Remove _get_poll lookup in Message constructor

This was triggering a terrible performance regression for no good
reason for all created messages that didn't have a poll, which is
essentially 99.99% of messages leading to MESSAGE_CREATE dispatches
having degraded performance.

discord/message.py

@@ -1849,7 +1849,7 @@ class Message(PartialMessage, Hashable):
         try:
             self.poll = Poll._from_data(data=data['poll'], message=self, state=state)
         except KeyError:
-            self.poll = state._get_poll(self.id)
+            pass
 
         try:
             # if the channel doesn't have a guild attribute, we handle that