This reverts commit 6beed8d5.

Reason for revert: Breaks some targets that lack a dependency onto v8-gn.h, see https://crbug.com/1178409.


Original change's description:
> [build] Remove V8_GN_HEADER hybrid mode
>
> v8_generate_external_defines_header has been enabled for a couple of
> weeks now with no problems, so this removes the hybrid state where
> V8_GN_HEADER was defined on the command line along with the other
> defines.
>
> Bug: v8:11292, v8:11341
> Change-Id: Ie78f00afe5d1cbca751101a7beb61a18c7d86d04
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2681947
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Auto-Submit: Dan Elphick <delphick@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72597}

TBR=mlippautz@chromium.org,delphick@chromium.org

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

No-try: true
Bug: v8:11292
Bug: v8:11341
Bug: chromium:1178409
Change-Id: Ic730ae1bdfe09b13cc0bec96a092822b1874fb0f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2692702
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72731}