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).

We could split this into 2 merge requests:

1. Remove feature flag use_legacy_pipeline_triggers and its use. This was added as a safe step to deprecate the feature. We moved it behind feature flag so that if removing the feature was causing errors or customer complains we could turn on the feature flag and it was behaving like before. We can now remove the feature entirely be removing what's behind the feature flag. For code to remove please refer to this MR gitlab-foss!30133 (merged), where the feature flag was introduced.
2. Create a db migration to drop all pipeline triggers that are "legacy" (having no owner)