filelog: stop proxying datafile (API)

It appears the censor code was the last user of this proxy. With
there being a dedicated censor API, we can drop the proxy.

Differential Revision: https://phab.mercurial-scm.org/D4660

mercurial/filelog.py

@@ -201,10 +201,6 b' class filelog(object):'
         self._revlog.indexfile = value
 
     @property
-    def datafile(self):
-        return self._revlog.datafile
-
-    @property
     def opener(self):
         return self._revlog.opener