This was left over from the previous CL to change S128LoadMem/S128StoreMem to
use prefixed opcodes. Decoding prefixed opcodes already checks for the
prototype flag.

BUG=V8:6020

R=bbudge@chromium.org

Review-Url: https://codereview.chromium.org/2946303002
Cr-Commit-Position: refs/heads/master@{#46108}