Revert "X86: support Swift Async context"

This reverts commit 747e5cfb.

Reason: New frame layout broke the sanitizer unwinder. Not clear why,
but seems like some of the changes aren't always guarded by Swyft
checks. See
https://reviews.llvm.org/rG747e5cfb9f5d944b47fe014925b0d5dc2fda74d7 for
more information.