Make a libsvn_ra_serf api that used to be in use using DELETE processing
static.

* subversion/libsvn_ra_serf/merge.c
  (svn_ra_serf__merge_lock_token_list): Rename to ...
  (merge_lock_token_list): ... this.
  (create_merge_body): Update caller.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__merge_lock_token_list): Remove prototype.

* subversion/mod_dav_svn/repos.c
  (remove_resource): Tweak comment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553966 13f79535-47bb-0310-9956-ffa450edef68