On the fsx-id branch: Some code cleanup.

* subversion/libsvn_fs_x/dag.h
  (svn_fs_x__dag_get_fs_id): Remove.

* subversion/libsvn_fs_x/dag.c
  (svn_fs_x__dag_get_fs_id): Remove.

* subversion/libsvn_fs_x/tree.h
  (svn_fs_x__node_id): Remove from header.

* subversion/libsvn_fs_x/tree.c
  (svn_fs_x__node_id): Rename to ...
  (x_node_id): ... this.  Construct the FS API ID here.
  (root_vtable): Update function reference.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1648125 13f79535-47bb-0310-9956-ffa450edef68