Previously AuthenticationMethod type was used in many places. Removed
all dependencies on it except from NegotiatingAuthenticator classes.
AuthenticationMethod has been moved to
NegotiatingAuthenticatorBase::Method.

Also includes the following cleanups:
 - removed old authentication functions from auth_util.cc which were
   no longer needed.
 - Remove HashFunction enum as it wasn't useful.

BUG=589698

Review URL: https://codereview.chromium.org/1768383004

Cr-Commit-Position: refs/heads/master@{#380078}