Pub/Sub: samples audit and cleanup

In addition to #1178, please also add:

1. `pubsub_enable_subscription_ordering`: [docs](https://cloud.google.com/pubsub/docs/ordering#enabling_message_ordering)
1. `pubsub_list_topic_subscriptions`: [docs](https://cloud.google.com/pubsub/docs/admin#listing_subscriptions), this is different from listing project subscriptions.
1. `pubsub_publisher_concurrency_control`: [docs](https://cloud.google.com/pubsub/docs/publisher#concurrency). This is optional, depending on if concurrency control even makes sense in PHP.
1. `pubsub_publisher_retry_settings`: [docs](https://cloud.google.com/pubsub/docs/publisher#retry)
1. `pubsub_publish_custom_attributes`: [docs](https://cloud.google.com/pubsub/docs/publisher#using_attributes)
1. `pubsub_publish_with_error_handler`: [docs](https://cloud.google.com/pubsub/docs/publisher#publishing_messages). The existing sample doesn't show how to handle publish failures. It's a quickstart publish sample.
1. `pubsub_publish_with_ordering_keys`: [docs](https://cloud.google.com/pubsub/docs/publisher#using_ordering_keys)
1. `pubsub_resume_publish_with_ordering_keys`: [docs](https://cloud.google.com/pubsub/docs/publisher#retry). There are two code includes in this section. Resume publish is towards the end of the section and shows the `resume_publish` method.
1. `pubsub_subscriber_async_pull`: [docs](https://cloud.google.com/pubsub/docs/pull#asynchronous-pull). Does this make sense in PHP?
1. `pubsub_subscriber_async_pull_custom_attributes`: [docs](https://cloud.google.com/pubsub/docs/pull#pull-messages-with-custom-attributes). Does this make sense in PHP?
1. `pubsub_subscriber_error_listener`: [docs](https://cloud.google.com/pubsub/docs/pull#error-listener). This is optional. The Java sample is the best example. Note that this method may not exist in PHP.
1. `pubsub_subscriber_flow_settings`: [docs](https://cloud.google.com/pubsub/docs/pull#config)
1. `pubsub_subscriber_sync_pull_with_lease`: [docs](https://cloud.google.com/pubsub/docs/pull#synchronous_pull_with_lease_management)

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.