[cleanup] Remove legacy async stepping support.
[cleanup] Remove legacy async stepping support.

This removes support for the legacy async stepping which has been
deprecated since V8 7.9 and is now officially dead code as Node 12 went
out of LTS last month.

Fixed: chromium:1287947
Bug: chromium:1000475
Change-Id: Ifb4319bccefa9466dc6a7cb236b2272a765c5311
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3620358
Reviewed-by: Kim-Anh Tran <a72f38a79b5fac6811022b8a7fb84830d856c9c9@chromium.org>
Auto-Submit: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>
Commit-Queue: Kim-Anh Tran <a72f38a79b5fac6811022b8a7fb84830d856c9c9@chromium.org>

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