Remove importer usage ping feature flag

- track importer events for github

Changelog: added

config/feature_flags/development/track_importer_activity.yml

----
-name: track_importer_activity
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70012
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339392
-milestone: '14.4'
-type: development
-group: group::import
-default_enabled: false

lib/gitlab/usage_data_counters/known_events/importer_events.yml

@@ -4,14 +4,11 @@
   category: importer
   redis_slot: import
   aggregation: weekly
-  feature_flag: track_importer_activity
 - name: github_import_project_success
   category: importer
   redis_slot: import
   aggregation: weekly
-  feature_flag: track_importer_activity
 - name: github_import_project_failure
   category: importer
   redis_slot: import
   aggregation: weekly
-  feature_flag: track_importer_activity