Remove the old (experimental) backup API.

The POST-based state API endpoint turned out to be incorrect.  Furthermore,
no client method was ever added to make use of the server-side method.
Merge pull request #492 from ericsnowcurrently/1.20-remove-old-backup

Remove the old (experimental) backup API.

The POST-based state API endpoint turned out to be incorrect.  Furthermore,
no client method was ever added to make use of the server-side method.

(same PR against master: https://github.com/juju/juju/pull/482)