Disable at_exit handlers

The [final step](https://github.com/rust-lang/rust/pull/19654) of
runtime removal changes the threading/process model so that the process
shuts down when the main thread exits. But several shared resources,
like the helper thread for timeouts, are shut down when the main thread
exits (but before the process ends), and they are not prepared to be
used after shut down, but other threads may try to access them during
the shutdown sequence of the main thread.

As an interim solution, the `at_exit` cleanup routine is simply skipped.

Ultimately, these resources should be made to safely handle asynchronous
shutdown, usually by panicking if called from a detached thread when the
main thread is ending.

See issue for details https://github.com/rust-lang/rust/issues/20012

This is a [breaking-change] for anyone relying on `at_exit`.

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