Factor out some repeated code.

* subversion/libsvn_wc/conflicts.c
  (remove_artifact_file_if_exists): New function, factored out ...
  (resolve_text_conflict_on_node,
   resolve_prop_conflict_on_node): ... from here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1477294 13f79535-47bb-0310-9956-ffa450edef68