Remove userbot functionality

This has a lot of legacy and cruft so there may be some stuff I've
missed but this first pass is enough to get a clear separation.

discord/webhook.py

@@ -403,10 +403,6 @@ class _PartialWebhookState:
     def store_user(self, data):
         return BaseUser(state=self, data=data)
 
-    @property
-    def is_bot(self):
-        return True
-
     @property
     def http(self):
         if self.parent is not None: