Remove i_testing_metrics_report_artifact_uploaders feature flag

Changelog: added

config/feature_flags/development/usage_data_i_testing_metrics_report_artifact_uploaders.yml

----
-name: usage_data_i_testing_metrics_report_artifact_uploaders
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51670
-rollout_issue_url:
-milestone: '13.9'
-type: development
-group: group::testing
-default_enabled: true

lib/gitlab/usage_data_counters/known_events/common.yml

@@ -179,7 +179,6 @@
   category: testing
   redis_slot: testing
   aggregation: weekly
-  feature_flag: usage_data_i_testing_metrics_report_artifact_uploaders
 - name: i_testing_summary_widget_total
   category: testing
   redis_slot: testing