Pub/Sub: samples audit and cleanup

[anguillanneuf]

In addition to #1178, please also add:

1. pubsub_enable_subscription_ordering: docs
2. pubsub_list_topic_subscriptions: docs, this is different from listing project subscriptions.
3. pubsub_publisher_concurrency_control: docs. This is optional, depending on if concurrency control even makes sense in PHP.
4. pubsub_publisher_retry_settings: docs
5. pubsub_publish_custom_attributes: docs
6. pubsub_publish_with_error_handler: docs. The existing sample doesn't show how to handle publish failures. It's a quickstart publish sample.
7. pubsub_publish_with_ordering_keys: docs
8. pubsub_resume_publish_with_ordering_keys: docs. There are two code includes in this section. Resume publish is towards the end of the section and shows the resume_publish method.
9. pubsub_subscriber_async_pull: docs. Does this make sense in PHP?
10. pubsub_subscriber_async_pull_custom_attributes: docs. Does this make sense in PHP?
11. pubsub_subscriber_error_listener: docs. This is optional. The Java sample is the best example. Note that this method may not exist in PHP.
12. pubsub_subscriber_flow_settings: docs
13. pubsub_subscriber_sync_pull_with_lease: docs

Remove:

1. Remove the region tag pubsub-publish but keep the other region tag pubsub_quickstart_publisher and keep the sample.

The PR to fix this will probably be big. Feel free to break it into logical chunks.