Delete unused header files.

This has been unused at least since Subversion 1.6, probably much longer.

* subversion/svn/client_errors.h
  subversion/svn-bench/client_errors.h
  Delete.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1625508 13f79535-47bb-0310-9956-ffa450edef68