Follow-up for #3017935: Media entities should no longer have path alias support by default

In #2835861: Discussion: Expose a path field for media entities → path alias support was added to Media entities, as they were visible on media/{id}
In #3017935: Media items should not be available at /media/{id} to users with 'view media' permission by default → that path was removed, and the canonical set to /media/{id}/edit with an option to restore the old canonical path.

But by default, Media has no publicly available canonical path. So, by default, path aliases shouldn't be activated for Media. It's cluttering the UX for the 80% use case.