Follow-up to r1643139: Fix pool cleanup loophole.

* subversion/libsvn_fs_fs/caching.c
  (remove_txn_cache_txn,
   remove_txn_cache_fs): Since we always register both cleanups, we must
                         always cleanup the respective other - even if the
                         cache object itself has already been removed. 

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1643233 13f79535-47bb-0310-9956-ffa450edef68