Remove `slack_events_app_home_opened` feature flag

What does this MR do and why?

This MR removes the slack_events_app_home_opened feature flag to enable the Slack App Home feature #363635 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #371678 (closed)