This reverts commit 189c8263.

Reason for revert: Lots of layout test changes:
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/16607

Original change's description:
> Enable StringConcat bytecode.
> 
> BUG=v8:6243
> 
> Change-Id: Idb9aa8221248cc493c8f5ba4bd50fa712a57f5f8
> Reviewed-on: https://chromium-review.googlesource.com/541221
> Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46249}

TBR=rmcilroy@chromium.org,jgruber@chromium.org,leszeks@chromium.org

Change-Id: I3ce96264d8c0e183b02c0344b90275e207683f7a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6243
Reviewed-on: https://chromium-review.googlesource.com/550157Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46261}