Cleanup group_protected_branches feature flag

See merge request !170128



Merged-by: Joe Woodward <jwoodward@gitlab.com>
Approved-by: Joe Woodward <jwoodward@gitlab.com>
Approved-by: Max Fan <mfan@gitlab.com>
Reviewed-by: Niklas van Schrick <mc.taucher2003@gmail.com>
Reviewed-by: Max Fan <mfan@gitlab.com>
Co-authored-by: Emma Park <epark@gitlab.com>