Drop mypy test suite? #5540

Continuation of #4333, also recently mentioned in #5303. The mypy test suite is becoming more and more of a liability for typeshed. In the last few days it failed twice for reasons outside the control of typeshed. This is causing unrelated PRs to fail. I also don't see the benefit of this test for typeshed, except to decrease the integration burden for mypy when syncing typeshed. But at this point, typeshed is used by at least four other type checkers, and typeshed is not as deeply integrated into mypy as it used to be. I don't think the trade off of keeping the test in typeshed is still worth it.