ref(alerts): Remove slack-compact-alerts feature flag

Unregister the organizations:slack-compact-alerts feature flag from
temporary features as part of compact alerts cleanup.

Co-Authored-By: Claude Opus 4.6 <noreply@example.com>

Author: leeandher

src/sentry/features/temporary.py

@@ -324,8 +324,6 @@ def register_temporary_features(manager: FeatureManager) -> None:
     manager.add("organizations:autofix-on-explorer-v2", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable Seer Workflows in Slack
     manager.add("organizations:seer-slack-workflows", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
-    # Enable new compact issue alert UI in Slack
-    manager.add("organizations:slack-compact-alerts", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable Seer Explorer in Slack via @mentions
     manager.add("organizations:seer-slack-explorer", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable search query attribute validation