Merge branch '425979-drop-index_namespaces_on_runners_token' into 'master'

Drop an unused index_namespaces_on_runners_token

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/132836



Merged-by: Simon Tomlinson <stomlinson@gitlab.com>
Approved-by: Simon Tomlinson <stomlinson@gitlab.com>
Co-authored-by: Krasimir Angelov <kangelov@gitlab.com>