Remove unnecessary while loop

This isn't needed as we turn asynchronous delivery off so the loop will
only be entered if the delivery fails and that causes an infinite loop!

tests/test_sessiontracker.py

@@ -1,4 +1,3 @@
-import time
 import logging
 import platform
 
@@ -83,8 +82,8 @@ def test_session_middleware_attaches_session_to_event(self):
 
         client.session_tracker.start_session()
         client.notify(Exception("Test"))
-        while len(self.server.received) == 0:
-            time.sleep(0.5)
+
+        assert len(self.server.received) == 1
 
         json_body = self.server.received[0]['json_body']
         session = json_body['events'][0]['session']