Remove an unused pool from a baton.

* subversion/svnserve/serve.c
  (fs_warning_baton_t): Remove pool.
  (fs_warning_func): Remove clearing.
  (construct_server_baton,
   reopen_repos): Update initialization.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558435 13f79535-47bb-0310-9956-ffa450edef68