1) Replace a condition by a DCHECK.
2) Remove dead argument.

R=thibaudm@chromium.org

Bug: v8:10321
Change-Id: I546d18e7e5520630ab32b4f73f9fe12b0441cb95
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2108556Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66790}