Revert of [builtins] Also migrate String.prototype.toLowerCase/toUpperCase to C++. (patchset #2 id:20001 of https://codereview.chromium.org/2018983002/ )

Reason for revert:
Please rebase blink first (if intended):
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/7092

Original issue's description:
> [builtins] Also migrate String.prototype.toLowerCase/toUpperCase to C++.
>
> These builtins always call into C++ anyways and so there's no point in
> having the JavaScript wrapper around them, but instead they can be
> implemented as C++ builtins directly.
>
> R=franzih@chromium.org
> BUG=v8:5049
>
> Committed: https://crrev.com/4e66888869bf04c73b41e5747e4595747a7b20df
> Cr-Commit-Position: refs/heads/master@{#36569}

TBR=franzih@chromium.org,bmeurer@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5049

Review-Url: https://codereview.chromium.org/2023753002
Cr-Commit-Position: refs/heads/master@{#36572}