Context

Describe the background behind the problem.
https://github.com/mongodb/mongo-python-driver/pull/2904 shows that the process for adding a new test suite and correctly having synchro convert changes is both non-obvious and poorly documented. This is partially due to the converted_tests list in tools/synchro.py that restricts which files in `test/asynchronous` are synchronized. This list serves no purpose now that all tests in `test/asynchronous` are synchronized, and so it should be removed.

Definition of done

What must be done to consider the task complete?
The synchro hook must automatically convert all files in test/asynchronous not explicitly excluded as async-only to synchronous versions. The CONTRIBUTING.md section "Converting a test to async" and tools/convert_test_to_async.py should also be removed.

Pitfalls

What should the implementer watch out for? What are the risks?