[wasm-simd] Remove i8x16.mul

This instruction is not in the final SIMD proposal.

Bug: v8:6020
Change-Id: Ifef1b3d58bf660f2d30784f587aed85f327825ec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2716073


Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73058}