XMLStream: Remove the process() helper method

The most common help request is about integration with other libraries,
and hiding our asyncio loop behind a very unhelpful `xmpp.process()`
isn’t a good way to guide users towards that.

Instead, let’s expose the gory asyncio details in every single example
and documentation, so that it becomes clear that we aren’t doing
anything special there.
3 jobs for bye-bye-xmpp-process in 5 minutes and 10 seconds (queued for 2 seconds)