In FSFS, remove the now unused DAG cache locking logic but don't update
the function signatures, yet.

* subversion/libsvn_fs_fs/tree.c
  (cache_lock_t): Drop.
  (fs_fs_dag_cache_t): Remove lock support and update docstring.
  (unlock_cache,
   unregister_locks): Remove unused pool cleanup callbacks.
  (svn_fs_fs__create_dag_cache): No cleanup handling needed anymore.
  (lock_cache): Remove unused function.
  (auto_clear_dag_cache): No locks to check for.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648539 13f79535-47bb-0310-9956-ffa450edef68