Merge branch '474647-remove-mixin-btn-primary-from-buttons-scss' into 'master'

Resolve "Remove @mixin btn-primary from buttons.scss"

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/161002



Merged-by: Enrique Alcántara <ealcantara@gitlab.com>
Approved-by: Enrique Alcántara <ealcantara@gitlab.com>
Co-authored-by: James Rushford <jrushford@gitlab.com>