Disable the polyfill in simd.js tests as the functions for Phase 1 have been implemented.

BUG=v8:4124
LOG=N

R=bbudge@chromium.org, bmeurer@chromium.org, littledan@chromium.org

Review URL: https://codereview.chromium.org/1305923005

Cr-Commit-Position: refs/heads/master@{#30452}