Remove child-process workers

[novemberborn]

I'm considering removing support for child-process workers. This means test files can only be run in worker threads.

The benefits are the potential use of shared memory and broadcast channels for AVA internals.

The main downside I can think of is projects who have edge case scenarios where the code being tested relies on a Node.js flag that is incompatible with worker threads. They'll no longer be able to use AVA.

If you think your use case is not an edge case, please comment so we can have a better understanding.