Since r1555138, the node_references member in the FSFS pack context
is no longer needed.  Remove it and all writes to it.

* subversion/libsvn_fs_fs/pack.c
  (pack_context_t): We only need the rep_references member and drop
                    its prefix.
  (initialize_pack_context,
   reset_pack_context): Update constructor and cleanup.
  (copy_rep_to_temp): Follow rename.
  (copy_node_to_temp): Don't write the node_references anymore.
  (compare_references_to): Drop.
  (compare_references_from,
   compare_ref_to_item_from): Rename to ...
  (compare_references,
   compare_ref_to_item): ... these.
  (sort_reps): Adapt to renames and remove node_references code.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555293 13f79535-47bb-0310-9956-ffa450edef68