Remove flaky PubSub snapshots system test

The PubSub backend does not give any guarantees about when a message
will be re-delivered after seeking back to a snapshot, it will only
be delivered "eventually". That causes flakiness in the snapshots
test.

Since the test cannot wait for an indefinite amount of time, this
commit removes it in order to not randomly break the CI builds.

pubsub/tests/system.py

@@ -297,48 +297,6 @@ def test_listing_topic_subscriptions(publisher, subscriber, project, cleanup):
     assert subscriptions == {subscription_paths[0], subscription_paths[2]}
 
 
-def test_using_snapshots(
-    publisher, subscriber, project, topic_path, subscription_path, cleanup
-):
-    snapshot_path = subscriber.snapshot_path(project, "my-snapshot")
-
-    # Make sure the topic, subscription, and snapshot get deleted.
-    cleanup.append((publisher.delete_topic, topic_path))
-    cleanup.append((subscriber.delete_subscription, subscription_path))
-    cleanup.append((subscriber.delete_snapshot, snapshot_path))
-
-    # publish a message to a topic, pull and acknowledge it
-    publisher.create_topic(topic_path)
-    subscriber.create_subscription(subscription_path, topic_path)
-
-    publisher.publish(topic_path, b"Message 1")
-    response = subscriber.pull(subscription_path, max_messages=100)
-    assert len(response.received_messages) == 1
-    msg = response.received_messages[0]
-    subscriber.acknowledge(subscription_path, [msg.ack_id])
-
-    # repeat the same *after* creating a snapshot
-    subscriber.create_snapshot(snapshot_path, subscription_path)
-
-    publisher.publish(topic_path, b"Message 2")
-    response = subscriber.pull(subscription_path, max_messages=100)
-    msg = response.received_messages[0]
-    subscriber.acknowledge(subscription_path, [msg.ack_id])
-
-    # pulling again, there should be no messages received
-    response = subscriber.pull(
-        subscription_path, max_messages=100, return_immediately=True
-    )
-    assert len(response.received_messages) == 0
-
-    # after seeking to the snapshot, the second message should be received again
-    subscriber.seek(subscription_path, snapshot=snapshot_path)
-    response = subscriber.pull(subscription_path, max_messages=100)
-    assert len(response.received_messages) == 1
-    msg = response.received_messages[0]
-    assert msg.message.data == b"Message 2"
-
-
 def test_managing_topic_iam_policy(publisher, topic_path, cleanup):
     cleanup.append((publisher.delete_topic, topic_path))