chore: remove SlackMessage._channel_id and SlackMessage.organization fields (#5340)

# What this PR does

Follow-up cleanup PR for https://github.com/grafana/oncall/pull/5325
(and https://github.com/grafana/oncall/pull/5330)

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/slack/0013_remove_slackmessage__channel_id_db.py

@@ -0,0 +1,22 @@
+# NOTE: this is being left in this directory on purpose, it will be moved to apps/alerts/migrations
+# in a separate PR/release
+#
+# Generated by Django 4.2.17 on 2024-12-06 17:05
+
+from django.db import migrations
+
+import common.migrations.remove_field
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("slack", "0012_remove_slackmessage_organization_state"),
+    ]
+
+    operations = [
+        common.migrations.remove_field.RemoveFieldDB(
+            model_name="SlackMessage",
+            name="_channel_id",
+            remove_state_migration=("slack", "0011_remove_slackmessage__channel_id_state"),
+        ),
+    ]

engine/apps/slack/0014_remove_slackmessage_organization_db.py

@@ -0,0 +1,22 @@
+# NOTE: this is being left in this directory on purpose, it will be moved to apps/alerts/migrations
+# in a separate PR/release
+#
+# Generated by Django 4.2.17 on 2024-12-06 17:05
+
+from django.db import migrations
+
+import common.migrations.remove_field
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("slack", "0013_remove_slackmessage__channel_id_db"),
+    ]
+
+    operations = [
+        common.migrations.remove_field.RemoveFieldDB(
+            model_name="SlackMessage",
+            name="organization",
+            remove_state_migration=("slack", "0012_remove_slackmessage_organization_state"),
+        ),
+    ]

engine/apps/slack/migrations/0010_remove_slackmessage_active_update_task_id_db.py

@@ -1,6 +1,3 @@
-# NOTE: this is being left in this directory on purpose, it will be moved to apps/alerts/migrations
-# in a separate PR/release
-#
 # Generated by Django 4.2.16 on 2024-12-04 12:00
 
 from django.db import migrations
@@ -10,7 +7,7 @@ import common.migrations.remove_field
 
 class Migration(migrations.Migration):
     dependencies = [
-        ("slack", "0008_remove_slackmessage_active_update_task_id_state"),
+        ("slack", "0009_drop_orphaned_messages_and_fill_in_missing_team_identity_values"),
     ]
 
     operations = [

engine/apps/slack/migrations/0011_remove_slackmessage__channel_id_state.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.17 on 2024-12-06 17:05
+
+import common.migrations.remove_field
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('slack', '0010_remove_slackmessage_active_update_task_id_db'),
+    ]
+
+    operations = [
+        common.migrations.remove_field.RemoveFieldState(
+            model_name='SlackMessage',
+            name='_channel_id',
+        ),
+    ]

engine/apps/slack/migrations/0012_remove_slackmessage_organization_state.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.17 on 2024-12-06 17:05
+
+import common.migrations.remove_field
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('slack', '0011_remove_slackmessage__channel_id_state'),
+    ]
+
+    operations = [
+        common.migrations.remove_field.RemoveFieldState(
+            model_name='SlackMessage',
+            name='organization',
+        ),
+    ]

engine/apps/slack/models/slack_message.py

@@ -39,11 +39,6 @@ class SlackMessage(models.Model):
     id = models.CharField(primary_key=True, default=uuid.uuid4, editable=False, max_length=36)
     slack_id = models.CharField(max_length=100)
 
-    _channel_id = models.CharField(max_length=100, null=True, default=None)
-    """
-    DEPRECATED/TODO: this is no longer being referenced/set, drop in a separate PR/release
-    """
-
     channel = models.ForeignKey(
         "slack.SlackChannel", on_delete=models.CASCADE, null=True, default=None, related_name="slack_messages"
     )
@@ -52,20 +47,6 @@ class SlackMessage(models.Model):
     (a slack message always needs to have a slack channel associated with it)
     """
 
-    organization = models.ForeignKey(
-        "user_management.Organization",
-        on_delete=models.CASCADE,
-        null=True,
-        default=None,
-        related_name="slack_message",
-    )
-    """
-    DEPRECATED/TODO: drop this field in a separate PR/release
-
-    For alert group related slack messages, we can always get the organization from the alert_group.channel
-    For shift swap request related slack messages, the schedule has a reference to the organization
-    """
-
     _slack_team_identity = models.ForeignKey(
         "slack.SlackTeamIdentity",
         on_delete=models.PROTECT,