Remove erroneous popover arrow color overrides

The following discussion from !36128 (merged) should be addressed:

• @psimyn started a discussion:

  note for future cleanup: we can remove all these blocks and set $popover-arrow-outer-color in https://gitlab.com/gitlab-org/gitlab/-/blob/3fb56f242f07deb7be70acd4ce2b34e9985fc0e2/app/assets/stylesheets/framework/variables_overrides.scss#L31 (or set $shadow-color in variables.scss directly)