Revert "Make Distributed.Worker threadsafe [Take 2]"

[vtjnash]

Reverts #38134, because we see via the rate of CI failures that Distributed is not ready to be threadsafe.

[jonas-schulze]

I would again suggest to remove the tests (or mark them otherwise as being flaky, maybe a new @test_flaky that differs from @test_broken in that it only logs a passing test). That way we can at least make Distributed threadsafe by parts / in baby steps. As I said before, the issue has very likely nothing to do with the changes you are reverting, rather than the tests that discover new problems.

[vtjnash]

Please don't feel discourage by this. It causes problems to leave CI in an unstable state for any length of time. However, that doesn't mean I'm opposed in any way to seeing this work progressed, just that it seems like there's more to be done before Distributed will support being run on a multi-threaded julia!