Enable service ping for error tracking features

Removes track_error_tracking_activity feature flag. Tracks counts of
unique users who visit the error tracking list or individual errors.

Changelog: added

config/feature_flags/development/track_error_tracking_activity.yml

----
-name: track_error_tracking_activity
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/82543
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/355112
-milestone: '14.9'
-type: development
-group: group::respond
-default_enabled: false

lib/gitlab/usage_data_counters/known_events/error_tracking.yml

@@ -3,9 +3,7 @@
   category: error_tracking
   redis_slot: error_tracking
   aggregation: weekly
-  feature_flag: track_error_tracking_activity
 - name: error_tracking_view_list
   category: error_tracking
   redis_slot: error_tracking
   aggregation: weekly
-  feature_flag: track_error_tracking_activity