The {msg} argument to Assembler::stop is dead since
https://crrev.com/2178093003 (July 2016). This CL removes it.

R=mstarzinger@chromium.org

Bug: v8:9396
Change-Id: I1593361709ab4977760f1ea21e3008797ef99cab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1692925
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62623}