Follow-up to r35236, fix a svnmerge.py regression.
Follow-up to r35236, fix a svnmerge.py regression.

* contrib/client-side/svnmerge/svnmerge.py
  (set_props): Don't attempt to delete non-existant properties.  Since
  issue #2220 was fixed in r35326 that is now an error.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion