Remove _pytest.compat.is_generator() and just use inspect.isgeneratorfunction() directly

[Zac-HD]

It's already equivalent in practice, and deleting the few uses and handful of tests will make it easier for contributors to understand code which uses the stdlib function instead.

While we're there, I'd like to turn the xfail on "yield tests" - a noop since Pytest 4! - into a hard error, to reduce the risk that users incorrectly believe that they're testing something.