Revert r1516453: serf reports cumulative progress information when used 
properly.

* subversion/libsvn_ra_serf/ra_serf.h
* subversion/libsvn_ra_serf/serf.c
  (svn_ra_serf__session_t, svn_ra_serf__progress): Revert r1516453.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1516466 13f79535-47bb-0310-9956-ffa450edef68