Remove feature flag :use_legacy_pipeline_triggers
In https://gitlab.com/gitlab-org/gitlab-ce/issues/30231 we moved the use of legacy triggers behind use_legacy_pipeline_triggers
feature flag and disabled on Gitlab.com.
Few releases after that we need to remove the feature flag and any traces of legacy triggers in the codebase.
We could also remove with a migration all remaining legacy triggers (triggers without owner).