downgrade: don't assume existence of nodemap files when downgrading

Persistent nodemap files might not exist on a fresh repository, so ignore the
potential missing file errors.

Differential Revision: https://phab.mercurial-scm.org/D11806

mercurial/revlogutils/nodemap.py

@@ -147,7 +147,7 b' def delete_nodemap(tr, repo, revlog):'
     if revlog._nodemap_file is None:
         msg = "calling persist nodemap on a revlog without the feature enabled"
         raise error.ProgrammingError(msg)
-    repo.svfs.unlink(revlog._nodemap_file)
+    repo.svfs.tryunlink(revlog._nodemap_file)
 
 
 def persist_nodemap(tr, revlog, pending=False, force=False):