Follow-up from "added new styling to the `.flash-warning` styles and moved the bundled style..."

The following discussion from gitlab-ce!28916 should be addressed:

• @winh started a discussion:

  I wonder if we should override $alert-border-radius from Bootstrap instead. We already have app/assets/stylesheets/framework/variables_overrides.scss for that purpose.

  However we can leave that as a follow-up issue.