Removes `track_epic_boards_activity` feature flag

Changelog: other

config/feature_flags/development/track_epic_boards_activity.yml

----
-name: track_epic_boards_activity
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60357
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/338038
-milestone: '13.12'
-type: development
-group: group::product planning
-default_enabled: true

lib/gitlab/usage_data_counters/known_events/epic_board_events.yml

@@ -7,16 +7,13 @@
   category: epic_boards_usage
   redis_slot: project_management
   aggregation: daily
-  feature_flag: track_epic_boards_activity
 
 - name: g_project_management_users_viewing_epic_boards
   category: epic_boards_usage
   redis_slot: project_management
   aggregation: daily
-  feature_flag: track_epic_boards_activity
 
 - name: g_project_management_users_updating_epic_board_names
   category: epic_boards_usage
   redis_slot: project_management
   aggregation: daily
-  feature_flag: track_epic_boards_activity