Remove (void) casts of ignored return values from ra_serf.

* subversion/libsvn_ra_serf/util.c
  (svn_ra_serf__conn_closed, svn_ra_serf__process_pending,
   svn_ra_serf__handle_xml_parser, svn_ra_serf__credentials_callback,
   svn_ra_serf__request_create, expat_start, expat_end,
   expat_cdata, expat_response_handler): Remove cast.

* subversion/libsvn_ra_serf/sb_bucket.c
  (sb_bucket_readline): Remove cast, clear error to avoid any possible leak.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464228 13f79535-47bb-0310-9956-ffa450edef68