Remove remaining occurences of tab.last_stanza_id

poezio/mam.py

@@ -147,7 +147,6 @@ async def retrieve_messages(tab: tabs.ChatTab,
     msg_count = 0
     msgs = []
     to_add = []
-    last_stanza_id = tab.last_stanza_id
     try:
         async for rsm in results:
             for msg in rsm['mam']['results']:

poezio/tabs/basetabs.py

@@ -492,8 +492,6 @@ class ChatTab(Tab):
         self._jid = jid
         #: Is the tab currently requesting MAM data?
         self.query_status = False
-        self.last_stanza_id = None
-
         self._name = jid.full  # type: Optional[str]
         self.text_win = windows.TextWin()
         self.directed_presence = None