perf: remove fallbacks to ancient versions of dirstate.walk()

If the call to dirstate.walk() failed, we would try to fall back to
older versions. These were removed in d3d1d39da2fa (walk: remove
cmdutil.walk, 2008-05-12) and f8299c84b5b6 (dirstate: fold statwalk
and walk, 2008-06-26). We don't care about testing performance of
versions that old versions at this point, so let's clean up.

Differential Revision: https://phab.mercurial-scm.org/D845

contrib/perf.py

@@ -370,15 +370,8 b' def clearfilecache(repo, attrname):'
 @command('perfwalk', formatteropts)
 def perfwalk(ui, repo, *pats, **opts):
     timer, fm = gettimer(ui, opts)
-    try:
-        m = scmutil.match(repo[None], pats, {})
-        timer(lambda: len(list(repo.dirstate.walk(m, [], True, False))))
-    except Exception:
-        try:
-            m = scmutil.match(repo[None], pats, {})
-            timer(lambda: len([b for a, b, c in repo.dirstate.statwalk([], m)]))
-        except Exception:
-            timer(lambda: len(list(cmdutil.walk(repo, pats, {}))))
+    m = scmutil.match(repo[None], pats, {})
+    timer(lambda: len(list(repo.dirstate.walk(m, [], True, False))))
     fm.end()
 
 @command('perfannotate', formatteropts)