Merge branch '474593-remove-mixin-btn-white-from-buttons-scss-part-2' into 'master'

Replace only instance of btn-white with contents of btn-white

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167730



Merged-by: Andrei Zubov <azubov@gitlab.com>
Approved-by: Andrei Zubov <azubov@gitlab.com>
Co-authored-by: James Rushford <jrushford@gitlab.com>