This reverts commit acadb202.

Reason for revert: chromium:941952

Original change's description:
> [turbofan] add fast path for String.p.startsWith
> 
> Add a fast path for String.p.startsWith(str) when length of str is 1.
> 
> Bug: v8:8400
> Change-Id: I65e657549902dc3ad064a213d815dd098ce6455f
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1491872
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#60091}

TBR=sigurds@chromium.org,bmeurer@chromium.org,usharma1998@gmail.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:8400
Change-Id: Iabd66b8db0d6958686ea15a042d8567955700c9e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1524205
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60279}