revlog: drop the df argument to `sidedata`

The intend of this argument is better filled by the `revlog.reading` or
`revlog._writing` context. So we drop it to leave rooms for further cleanup and
improvements.

mercurial/revlog.py

@@ -1960,7 +1960,7 b' class revlog:'
         """
         return self._revisiondata(nodeorrev)
 
-    def sidedata(self, nodeorrev, _df=None):
+    def sidedata(self, nodeorrev):
         """a map of extra data related to the changeset but not part of the hash
 
         This function currently return a dictionary. However, more advanced