Revert of [builtins] migrate C++ String Iterator builtins to baseline TurboFan (patchset #13 id:260001 of https://codereview.chromium.org/2358263002/ )

Reason for revert:
Introduces an infinite loop (see comment).

Original issue's description:
> [builtins] migrate C++ String Iterator builtins to baseline TurboFan
>
> Migrate newly added C++ String Iterator builtins to TFJ builtins, per
> step 4. of the String Iterator Baseline Implementation section of the design doc
>
> BUG=v8:5388
> R=bmeurer@chromium.org, mstarzinger@chromium.org
>
> Committed: https://crrev.com/f9a2c8b1112c4e915df8bc5f7ea1fccdf7a33ff8
> Cr-Commit-Position: refs/heads/master@{#39765}

TBR=bmeurer@chromium.org,mstarzinger@chromium.org,caitp@igalia.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:5388

Review-Url: https://codereview.chromium.org/2374123005
Cr-Commit-Position: refs/heads/master@{#39870}