Merge branch 'doc/glas-ff-removal' into 'master'

Remove mention to deleted feature flags

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179469



Merged-by: Russell Dickenson <rdickenson@gitlab.com>
Approved-by: Russell Dickenson <rdickenson@gitlab.com>
Co-authored-by: Thiago Figueiró <tfigueiro@gitlab.com>