Fix a compiler warning by removing some pointless code.

* subversion/libsvn_ra_serf/update.c
  (body_allocate_all): Remove test for sz >= 0, apr_size_t is not signed so it
    will always be true.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1563991 13f79535-47bb-0310-9956-ffa450edef68