Fix assertion error for messages_on_hold._activate_ordering_keys

[acocuzzo]

It is possible to call activate_ordering_keys with keys which are not present in messages_on_hold._pending_ordered_messages. This can happen if:

1. A message (or messages) lease exceeds max_lease_duration, it is dropped from maintain_leases

2. We call drop in the dispatcher

   python-pubsub/google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py

   Line 295 in 9ffb8d6

   def drop(

3. If there are no messages remaining in messages_on_hold._pending_ordered_messages, therefore the pending_ordered_messages queue for the ordering key is removed:
   https://github.com/googleapis/python-pubsub/blob/main/google/cloud/pubsub_v1/subscriber/_protocol/messages_on_hold.py#L154

4. The message is acked from the user callback, the ack completes and drop is called in the dispatcher:
   

   python-pubsub/google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py

   Line 247 in f3bf21d

   self.drop(requests_completed)

5. We call _activate_ordering_keys with a key that does not exist in messages_on_hold._pending_ordered_messages:
   https://github.com/googleapis/python-pubsub/blob/main/google/cloud/pubsub_v1/subscriber/_protocol/messages_on_hold.py#L122

Reproduction:

1. Publish a single message with an ordering key:
2. Subscribe with flow control max_lease_duration < time to ack the message, to ensure that the library will drop leasing prior to the message being acked.

Results:

Listening for messages...
Received b'message1'.
Dropping 1 items because they were leased too long.
Acking message with ordering key: key1 
AssertionError: A message queue should exist for every ordered message in flight.