simplemerge: delete unused find_unconflicted()
The function has been unused ever since it was introduced in
465b9ea02868 (Import 3-way merge code from bzr, 2007-04-16).
Differential Revision: https://phab.mercurial-scm.org/D9832
#require docutils gettext $ $TESTDIR/check-gendoc it checking for parse errors