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