A while back, we had added some additional debug logging to our auth

backends' authenticate() methods, in an effort to help provide useful

information if these methods ever got called with missing credentials.
That can be useful, but in practice it was causing spurious warnings

when an API client tried to use an API token to authenticate and when

using an auth backend that had this logging.
The API token web auth backend would generate credentials and then call

auth.authenticate(), which would loop through all auth backends until

one handled the request. This was intended to trigger a corresponding

API token auth backend (either the default in Djblets or the default in

Review Board).
This backend is registered after the normal auth backends, so we'd do

one pass through those backends, log the warnings, and then get to the

proper backends.
Ideally, we'd rework some of this to be able to consider only backends

matching certain criteria, but that'd require some larger rework of

this. For now, we're going to just remove those warnings, since they're

doing more harm than good.

Unit tests pass.