Disable revprop caching unconditionally for now.

* subversion/libsvn_fs_fs/caching.c
  (read_config): Always say "no" on revprop caching.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619774 13f79535-47bb-0310-9956-ffa450edef68