Sign extension opcodes are already tested in the interpreter since
https://crrev.com/c/1083337, thus remove the obsolete TODO.

R=gdeepti@chromium.org

No-Try: true
Change-Id: I08323b9d43fa46d1fcf9965e5106a68767f695b5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1954400
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65364}