Remove legacy avatar code

- Remove method that was only left around because the
  [api](https://github.com/discourse/discourse_api/pull/53) called it
- Modify test to use new route instead of legacy route

https://meta.discourse.org/t/legacy-route-for-avatars/22838/2
Merge pull request #3026 from oblakeerickson/legacy_avatar

Remove legacy avatar code