Revert r342652 "[winasan] Unpoison the stack in NtTerminateThread"

This seems to cause the thread's exit code to be clobbered, breaking
Chromium tests.

Also revert follow-up r342654.

> In long-running builds we've seen some ASan complaints during thread creation that we suspect are due to leftover poisoning from previous threads whose stacks occupied that memory. This patch adds a hook that unpoisons the stack just before the NtTerminateThread syscall.
>
> Differential Revision: https://reviews.llvm.org/D52091

llvm-svn: 343322