[d8] Remove GOOGLE3 main function guard

This guard is deprecated. We actually have floating patches in g3 that
removes these lines.

Change-Id: I11168b3ca1f3985039cf31ddef70d5b292de85b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2959619
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75122}

src/d8/d8.cc

@@ -5265,9 +5265,7 @@ int Shell::Main(int argc, char* argv[]) {
 
 }  // namespace v8
 
-#ifndef GOOGLE3
 int main(int argc, char* argv[]) { return v8::Shell::Main(argc, argv); }
-#endif
 
 #undef CHECK
 #undef DCHECK