fix: folder/*: never set self.messagelist to None

Empty the list by setting an empty dict.
Introduce BaseFolder().ismessagelistempty().

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/LocalStatus.py

@@ -155,9 +155,6 @@ class LocalStatusFolder(BaseFolder):
         self.readstatus(cachefd)
         cachefd.close()
 
-    def dropmessagelistcache(self):
-        self.messagelist = None
-
     def save(self):
         """Save changed data to disk. For this backend it is the same as saveall."""