Remove duplicate doc strings on public function definitions.

* subversion/libsvn_fsfs/cached_data.c
  (svn_fs_fs__get_changes): Remove doc string.

* subversion/libsvn_fs_x/cached_data.c
  (svn_fs_x__get_changes): Remove doc string.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1546970 13f79535-47bb-0310-9956-ffa450edef68