[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}

BUILD.gn

@@ -575,12 +575,10 @@ disabled_external_v8_defines = external_v8_defines - enabled_external_v8_defines
 config("v8_header_features") {
   visibility = [ ":*" ]
 
-  # TODO(v8:11292): Set defines as well as including the header. This is a short
-  # term change to check that the header doesn't get stale and fail to get
-  # updated.
-  defines = enabled_external_v8_defines
   if (v8_generate_external_defines_header) {
-    defines += [ "V8_GN_HEADER" ]
+    defines = [ "V8_GN_HEADER" ]
+  } else {
+    defines = enabled_external_v8_defines
   }
 }
 
@@ -610,12 +608,10 @@ disabled_external_cppgc_defines =
 config("cppgc_header_features") {
   visibility = [ ":*" ]
 
-  # TODO(v8:11292): Set defines as well as including the header. This is a short
-  # term change to check that the header doesn't get stale and fail to get
-  # updated.
-  defines = enabled_external_cppgc_defines
   if (v8_generate_external_defines_header) {
-    defines += [ "V8_GN_HEADER" ]
+    defines = [ "V8_GN_HEADER" ]
+  } else {
+    defines = enabled_external_cppgc_defines
   }
 }