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

hgext/beautifygraph.py

@@ -31,8 +31,6 b" testedwith = 'ships-with-hg-core'"
 def prettyedge(before, edge, after):
     if edge == '~':
         return '\xE2\x95\xA7' # U+2567 ╧
-    if edge == 'X':
-        return '\xE2\x95\xB3' # U+2573 ╳
     if edge == '/':
         return '\xE2\x95\xB1' # U+2571 ╱
     if edge == '-':