On the verify-keep-going branch, remove redundant code.

* subversion/libsvn_repos/dump.c
  (notify_verification_error, notify_verification_summary): These functions
   are only called if notify_func is not NULL, so they don't need to verify
   this argument.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/verify-keep-going@1546059 13f79535-47bb-0310-9956-ffa450edef68