Remove FF ci_variables_api_filter_environment_scope

While implementing !34490 (merged), we used feature flag ci_variables_api_filter_environment_scope for rolling out a breaking change.

In this issue, we need to remove ci_variables_api_filter_environment_scope completely.

• Remove the FF (!41680 (merged))
• Clean up https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up