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
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors