CI: Fix dumping GDExtension interface and API for godot-cpp

Follow-up to https://github.com/godotengine/godot-cpp/pull/960.

Fix exit code for --dump-extension-api and --dump-gdextension-interface.

Removed the planned API validation step as we still didn't implement
anything, and maintaining a stub isn't useful.

main/main.cpp

@@ -2594,6 +2594,7 @@ bool Main::start() {
 	}
 
 	if (dump_gdextension_interface || dump_extension_api) {
+		OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
 		return false;
 	}