Remove known limitation section from GitLab Operator docs

Closes #353

See merge request !598



Merged-by: Jason Plum <jplum@gitlab.com>
Approved-by: Jason Plum <jplum@gitlab.com>
Approved-by: Mitchell Nielsen <mnielsen@gitlab.com>
Co-authored-by: João Alexandre Cunha <j.a.cunha@gmail.com>