Revert of [wasm][asm.js] Enable --validate-asm by default. (patchset #2 id:20001 of https://codereview.chromium.org/2624813002/ )

Reason for revert:
Triggers flaky tests.

Original issue's description:
> [wasm][asm.js] Enable --validate-asm by default.
>
> This directs all asm.js traffic via the Wasm backend.
>
> R=titzer@chromium.org,aseemgarg@chromium.org
> BUG=v8:4203
>
> Review-Url: https://codereview.chromium.org/2624813002
> Cr-Original-Commit-Position: refs/heads/master@{#42194}
> Committed: https://chromium.googlesource.com/v8/v8/+/946cc371ed4b34d1a9f5cc615b14c41b652562ad
> Review-Url: https://codereview.chromium.org/2624813002
> Cr-Commit-Position: refs/heads/master@{#42244}
> Committed: https://chromium.googlesource.com/v8/v8/+/3169fb94c98953f002908974a606b51a35178046

TBR=aseemgarg@chromium.org,titzer@chromium.org,bradnelson@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4203

Review-Url: https://codereview.chromium.org/2627893003
Cr-Commit-Position: refs/heads/master@{#42253}