On the fsx-id branch: Remove a few unused ID functions and remove
a couple more from the header.

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__id_part_compare): Move up.
  (svn_fs_x__id_txn_id,
   svn_fs_x__id_rev,
   svn_fs_x__id_item,
   svn_fs_x__id_check_related,
   svn_fs_x__id_compare,
   svn_fs_x__id_txn_create_root,
   svn_fs_x__id_create_root,
   svn_fs_x__id_txn_create): Remove from header.

* subversion/libsvn_fs_x/id.c
  (svn_fs_x__id_txn_id,
   svn_fs_x__id_rev,
   svn_fs_x__id_item): Remove.
  (svn_fs_x__id_check_related: Rename to ...
  (id_check_related): ... this.
  (svn_fs_x__id_compare): Rename to ...
  (id_compare): ... this.  Update caller.
  (id_vtable): Update function reference.
  (svn_fs_x__id_txn_create_root,
   svn_fs_x__id_create_root,
   svn_fs_x__id_txn_create): Remove.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1648070 13f79535-47bb-0310-9956-ffa450edef68