This reverts commit 67960ba1.

Reason for revert:
This has been properly fixed by https://crrev.com/c/3053740.
Now dcheck_always_on already defaults to false for subprojects
like V8 and no other switch is required. The switch didn't fully
work anyways due to https://crbug.com/1231890.

Original change's description:
> Reland "[build] Add V8-specific dcheck_always_on"
>
> This is a reland of cecc666f
>
> Depends on:
> https://crrev.com/c/3043611
>
> Original change's description:
> > [build] Add V8-specific dcheck_always_on
> >
> > This makes the V8 dcheck control independent of Chromium's and
> > prepares switching Chromium's default behavior without affecting V8
> > developers or builders.
> >
> > Preparation for: https://crrev.com/c/2893204
> >
> > Bug: chromium:1225701
> > Change-Id: I520b96019b04196f4420716ff3500ebd6c21666f
> > Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3038528
> > Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> > Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#75827}
>
> Bug: chromium:1225701
> Change-Id: I56568b78592addba01793d2d14f768c9ee10103d
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3041670
> Reviewed-by: Liviu Rau <liviurau@chromium.org>
> Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#75839}

Bug: chromium:1225701, chromium:1231890
Change-Id: I7e27f5774d8e162977f30f685da4b15dadcc1084
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3055294Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75935}