Remove merge_when_checks_pass feature flag

app/models/merge_request.rb

app/services/auto_merge/merge_when_checks_pass_service.rb

...rvices/auto_merge/merge_when_pipeline_succeeds_service.rb

app/services/discussions/resolve_service.rb

app/services/merge_requests/post_merge_service.rb

app/services/merge_requests/update_service.rb

...rs/merge_requests/process_auto_merge_from_event_worker.rb

config/feature_flags/beta/merge_when_checks_pass.yml

doc/user/project/merge_requests/auto_merge.md

ee/app/models/merge_requests/status_check_response.rb

...auto_merge/add_to_merge_train_when_checks_pass_service.rb

ee/app/services/ee/merge_requests/post_merge_service.rb

ee/app/services/ee/merge_requests/update_service.rb

ee/app/services/merge_requests/update_blocks_service.rb

...ec/controllers/projects/merge_requests_controller_spec.rb

ee/spec/models/merge_request_spec.rb

ee/spec/models/merge_requests/status_check_response_spec.rb

ee/spec/serializers/merge_request_poll_widget_entity_spec.rb

...ervices/auto_merge/merge_when_checks_pass_service_spec.rb

...pec/services/ee/merge_requests/post_merge_service_spec.rb