manifest: remove check for non-contexts in _dirmancache

It looks like the _dirmancache has contained only manifest contexts
since d79c141fdf41 (manifest: remove usages of manifest.read,
2016-11-10).

mercurial/manifest.py

@@ -1340,12 +1340,7 b' class manifestlog(object):'
                    the revlog
         """
         if node in self._dirmancache.get(dir, ()):
-            cachemf = self._dirmancache[dir][node]
-            # The old manifest may put non-ctx manifests in the cache, so
-            # skip those since they don't implement the full api.
-            if (isinstance(cachemf, manifestctx) or
-                isinstance(cachemf, treemanifestctx)):
-                return cachemf
+            return self._dirmancache[dir][node]
 
         if dir:
             if self._revlog._treeondisk: