using 'tools/dev/warn-unused-result.sh'

Found

r1817814

r1817814 | julianfoad | 2017-12-11 19:58:02 +0000 (Mon, 11 Dec 2017)

* subversion/include/private/svn_element.h,
  subversion/libsvn_delta/element.c
  (svn_element__tree_set): Remove unused 'svn_error_t *' return.

Found by: danielsh
  using 'tools/dev/warn-unused-result.sh'