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}