On the fsx-id branch:  Remove the unused revision root ID cache.
This is a remnant of FSFS pre-format 7.

* subversion/libsvn_fs_x/fs.h
  (fs_x_data_t): Remove the cache instance.

* subversion/libsvn_fs_x/caching.c
  (svn_fs_x__initialize_caches):  One less to initialize.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1647518 13f79535-47bb-0310-9956-ffa450edef68