Fix conv.wait_event no longer raising Timeout

Should properly fix #1618.

telethon/tl/custom/conversation.py

@@ -338,10 +338,10 @@ class Conversation(ChatGetter):
         self._custom[counter] = (event, future)
         try:
             return await self._get_result(future, start_time, timeout, self._custom, counter)
-        except asyncio.TimeoutError:
+        finally:
             # Need to remove it from the dict if it times out, else we may
             # try and fail to set the result later (#1618).
-            del self._custom[counter]
+            self._custom.pop(counter, None)
 
     async def _check_custom(self, built):
         for key, (ev, fut) in list(self._custom.items()):