beautifygraph: don't substitute anything for 'X' in rendered graphs
It looks like we never actually render graphs this way, although there's an
example in a comment next to the code that prevents it from being needed (see
graphmod.ascii()).
Differential Revision: https://phab.mercurial-scm.org/D5103
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors