Revert of [wasm][asm.js] Enable --validate-asm by default. (patchset #1 id:1 of https://codereview.chromium.org/2624813002/ )

Reason for revert:
Breaks layout tests:
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/12554

See also:
https://github.com/v8/v8/wiki/Blink%20layout%20tests

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-Commit-Position: refs/heads/master@{#42194}
> Committed: https://chromium.googlesource.com/v8/v8/+/946cc371ed4b34d1a9f5cc615b14c41b652562ad

TBR=aseemgarg@chromium.org,titzer@chromium.org,bradnelson@google.com,bradnelson@chromium.org
# 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/2622023002
Cr-Commit-Position: refs/heads/master@{#42200}