Remove unused conflict_func/conflict_baton in file externals code.

* subversion/include/private/svn_wc_private.h
  (svn_wc__get_file_external_editor): Remove conflict_func/conflict_baton
   parameters.

* subversion/libsvn_client/externals.c
  (switch_file_external): Update caller.

* subversion/libsvn_wc/externals.c
  (edit_baton, svn_wc__get_file_external_editor): Remove unused conflict_func
   and conflict_baton.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618864 13f79535-47bb-0310-9956-ffa450edef68