Extern ref in Liftoff is enabled since M-90
(https://crrev.com/c/2625886), hence remove the flag to simplify the
code.

R=ahaas@chromium.org

Bug: v8:11879
Change-Id: Ie72dfbc006d6f42e2e9e83d44ff78e3c53a82614
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2996195Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75478}