manifest: remove unused property _oldmanifest

The last use seems to have gone away in 7c7d845f8b64 (manifest: make
manifestlog use it's own cache, 2016-11-10).

mercurial/manifest.py

@@ -1317,8 +1317,7 b' class manifestlog(object):'
             cachesize = opts.get('manifestcachesize', cachesize)
         self._treeinmem = usetreemanifest
 
-        self._oldmanifest = repo._constructmanifest()
-        self._revlog = self._oldmanifest
+        self._revlog = repo._constructmanifest()
 
         # A cache of the manifestctx or treemanifestctx for each directory
         self._dirmancache = {}