Cleanup graphql_git_blame feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143343



Merged-by: Nikola Milojevic <nmilojevic@gitlab.com>
Approved-by: Nikola Milojevic <nmilojevic@gitlab.com>
Approved-by: Radamanthus Batnag <rbatnag@gitlab.com>
Co-authored-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: ghinfey <ghinfey@gitlab.com>