Remove NO_THREADS fallback code, Godot 4 requires thread support

This also removes `OS::can_use_threads` from the public API since it's always
true.

modules/noise/noise_texture_2d.cpp

@@ -197,9 +197,6 @@ void NoiseTexture2D::_update_texture() {
 		use_thread = false;
 		first_time = false;
 	}
-#ifdef NO_THREADS
-	use_thread = false;
-#endif
 	if (use_thread) {
 		if (!noise_thread.is_started()) {
 			noise_thread.start(_thread_function, this);