Revert "core.thread.refactor Part 1 (#2821)"

[rainers]

This reverts commit 1d39caa.

trying to see whether it helps and the azure CI works better with a PR not using the dlang repo as in #2822

[dlang-bot]

Thanks for your pull request, @rainers!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + druntime#2825"

[rainers]

druntime back to green via #2823