Remove wiki front matter related feature flags

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/160557



Merged-by: Lukas Eipert <leipert@gitlab.com>
Approved-by: Kos Palchyk <kpalchyk@gitlab.com>
Approved-by: Roy Liu <rliu@gitlab.com>
Approved-by: Ravi Kumar <rkumar@gitlab.com>
Approved-by: Lukas Eipert <leipert@gitlab.com>
Reviewed-by: Lukas Eipert <leipert@gitlab.com>
Co-authored-by: Himanshu Kapoor <hkapoor@gitlab.com>