Revert "Allow `admin_runner` ability to change shared runners setting"

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/156955



Merged-by: Thong Kuah <tkuah@gitlab.com>
Approved-by: Stan Hu <stanhu@gmail.com>
Approved-by: Hinam Mehra <hmehra@gitlab.com>
Co-authored-by: mo khan <mo@mokhan.ca>