Docs: remove unused .bd-file-ref class

[julien-deramond]

This PR removes the use of .bd-file-ref class that doesn't have anymore a corresponding CSS rule.

In our main branch, we have this code allowing to have a clean rounded corner for some code snippets:

.bd-file-ref {
  .highlight-toolbar {
    @include media-breakpoint-up(md) {
      @include border-top-radius(calc(var(--bs-border-radius) - 1px));
    }
  }
}

Avoiding this:

In the v6-dev branch, this rendering is already handled by the @media (width >= 768px) { .highlight-toolbar:first-child CSS rule.