Follow-up to r1555294: Remove unused functions.

* subversion/libsvn_fs_x/cached_data.c
  (delta_read_next_window,
   delta_read_md5_digest): Remove.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555299 13f79535-47bb-0310-9956-ffa450edef68