Removed dav_get_limit_xml_body() from mod_dav.h.

This was a forgotten prototype hanging around for close
to 11 years where no code for existed (see r85816);
now removed from all branches per wrowe's permission.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1083536 13f79535-47bb-0310-9956-ffa450edef68