Remove some invalid documentation of completely unused defines. Too bad
we can't just remove them... Just mark them deprecated.

* subversion/include/svn_client.h
  (SVN_CLIENT_AUTH_USERNAME,
   SVN_CLIENT_AUTH_PASSWORD):
     Deprecate two defines from the original cvs code, which were
     unused since the introduction of the generic auh handling in 2003.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1624010 13f79535-47bb-0310-9956-ffa450edef68