Remove the old HTTP-based API server endpoint for backups.

The POST-based API has been rejected.

I'd also like to backport this change to 1.20.  The API endpoint for backups is "experimental" there and it would be good to have the POST-based version removed before 1.20 makes it into trusty.  Backporting the new backups implementation to 1.20 isn't trivial (or small) so simply removing it from 1.20 makes more sense to me.