In FSX, remove node origin detection legacy code carried over from FSFS.

* subversion/libsvn_fs_x/tree.c
  (prev_location): Drop obsolete function.
  (x_node_origin_rev): The creation rev is always given in FSX by the node_id.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567211 13f79535-47bb-0310-9956-ffa450edef68