Remove feature flag for only checking downstream release environments pipeline

Context

In #21167 (closed), we implemented a feature that was behind the feature flag. This feature makes release tooling determine stable branch status with downstream release environments pipeline, instead of the merge commit pipeline.

MR: gitlab-org/release-tools!4219 (merged)

We put it under the FF in order to continue without being blocked by Analysis: Job difference between pipelines on b... (#21002 - closed). The FF can be removed once DevEx confirms that the pre-merge and post-merge pipelines should behave the same.

Prior to that, if we have a suspicion of the patch release process having less coverage, the FF could be used to return the check to its original behaviour.

Exit Criteria

• Remove the feature flag stable_branch_check_release_environments_only that was implemented in gitlab-org/release-tools!4219 (merged)