app: [Metal] don't wait for completion of presentation command

There's no need to wait; the Metal backend performs the required
synchronization.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/metal_darwin.go

@@ -62,7 +62,6 @@ static void presentDrawable(CFTypeRef queueRef, CFTypeRef drawableRef) {
 		id<MTLCommandBuffer> cmdBuffer = [queue commandBuffer];
 		[cmdBuffer presentDrawable:drawable];
 		[cmdBuffer commit];
-		[cmdBuffer waitUntilCompleted];
 	}
 }