Remove 'emoji' property from Slack markdown blocks in resolution notes (#702)

I'm seeing this error in the logs:

> invalid additional property: emoji [json-pointer:/blocks/0/text]

It looks like the 'emoji' argument is not supported when type: mrkdwn.
This PR removes that property in the same place that the change
to mrkdwn was made - see [this
PR](https://github.com/grafana/oncall/pull/646).

engine/apps/slack/scenarios/resolution_note.py

@@ -358,7 +358,7 @@ class UpdateResolutionNoteStep(scenario_step.ScenarioStep):
         author_verbal = resolution_note.author_verbal(mention=True)
         resolution_note_text_block = {
             "type": "section",
-            "text": {"type": "mrkdwn", "text": resolution_note.text, "emoji": True},
+            "text": {"type": "mrkdwn", "text": resolution_note.text},
         }
         blocks.append(resolution_note_text_block)
         context_block = {