Code Cleanup for Attention Requests

Problem to Solve

As we'll no longer be rolling out this feature we should make sure to cleanup the codebase so that we don't impact future development negatively with legacy information.

Proposal

• Remove code from backend
• Remove code from frontend
• Remove all traces of related feature flags
  • uncached_mr_attention_requests_count #357480 (closed)
  • mr_attention_requests #343528 (closed)
• Rollback/remove unnecessary database changes