This reverts commit d1da9694.

Reason for revert: Compile error on msvc: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Win64%20-%20msvc/16456/overview

Original change's description:
> [build] Enable external flag header by default
>
> Turns on v8_generate_external_defines_header.
>
> Bug: v8:11292
> Change-Id: I4b1d9b47390b560b7cbf677948310694d8b03367
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2610966
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Commit-Queue: Dan Elphick <delphick@chromium.org>
> Auto-Submit: Dan Elphick <delphick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72201}

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

Change-Id: Ia0005704b9e3b33acc73c088d287e66a99360423
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:11292
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2640418Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72202}