Cleanup group_protected_branches feature flag

What does this MR do and why?

Related to [Feature flag] Cleanup group_protected_branches (#500250 - closed)

This issue is to cleanup the group_protected_branches feature flag. The group_protected_branches feature flag is the same as the allow_protected_branches_for_group feature flag. We only have the additional flag due to a naming conflict with a licensed feature. We kept it around in case anyone has manually enabled it so the feature didn’t suddenly disappear.

Now that allow_protected_branches_for_group has been already cleaned up, we should remove group_protected_branches feature flag as well.

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