remove 1-second join from threadexception plugin

[graingert]

          I think it is a reasonable thing for someone to run some thread unrelated to pytest, like a monitoring thread or similar (there are many which do this), and then run `pytest.main()`. If I understand correctly, this would then cause an unnecessary 1s delay to the pytest run. Best case, they report a bug, worst case they just think pytest is slow and suffer through it.

So IMO, unless we can somehow restrict join_threads only to threads started by tests, we shouldn't do it.

Originally posted by @bluetech in #13016 (comment)