remove AlertGroup.is_restricted column (#3287)

# What this PR does

Second part of https://github.com/grafana/oncall/pull/3256 to remove the
deprecated `AlertGroup.is_restricted` column. That change was released
in v1.3.54.

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

engine/apps/alerts/migrations/0038_remove_alertgroup_is_restricted_db.py

@@ -0,0 +1,21 @@
+# Generated by Django 4.2.6 on 2023-11-03 23:02
+
+import common.migrations.remove_field
+from django.db import migrations
+import django_migration_linter as linter
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('alerts', '0037_remove_alertgroup_is_restricted_state'),
+    ]
+
+    operations = [
+        linter.IgnoreMigration(),
+        common.migrations.remove_field.RemoveFieldDB(
+            model_name='AlertGroup',
+            name='is_restricted',
+            remove_state_migration=('alerts', '0037_remove_alertgroup_is_restricted_state'),
+        ),
+    ]