There remained a few of regressions and we didn't see any significant
improvement in the real world with this turned on. This CL reverts all the
StringConcat bytecode work which landed.

BUG=v8:6243

Change-Id: I832eb72e880ad41411dbec8fe29f71ef0f2025c8
Reviewed-on: https://chromium-review.googlesource.com/575130
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46769}