Revert "Reland "[build] Enable external flag header by default""

This reverts commit df5854c9.

Reason for revert: The v8-gn.h header file seems to be inconsistently rebuilt sometimes causing build failures: https://bugs.chromium.org/p/v8/issues/detail?id=11341

Original change's description:
> Reland "[build] Enable external flag header by default"
>
> This is a reland of d1da9694
>
> Relanding now that fixes for perfetto and cppgc are in:
> https://chromium-review.googlesource.com/c/v8/v8/+/2640458
> https://chromium-review.googlesource.com/c/v8/v8/+/2640480
>
> 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}
>
> Bug: v8:11292
> Change-Id: Ia47eeb6c45f4cc3db72c10782d677b69506fa3d6
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2642249
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Commit-Queue: Dan Elphick <delphick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72226}

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

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

Bug: v8:11292, v8:11341
Change-Id: I00719689e7a184e74699d074a01c10b4f462e553
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2647259Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72289}

BUILD.gn

@@ -288,7 +288,7 @@ declare_args() {
 
   # If enabled then macro definitions that are used in externally visible
   # header files are placed in a separate header file v8-gn.h.
-  v8_generate_external_defines_header = true
+  v8_generate_external_defines_header = false
 }
 
 # Derived defaults.