Fix compiler error in Object.dispose (return value goes unused)

[cherrythecool]

was messing around with objective-c stuff using this and found that if i called .dispose() on an object it wouldn't compile cuz c.object_dispose() returns a value

looking at the apple documentation it looks like the return value isn't important (it literally just says nil), so i just did a simple _ = to let it compile and free the object from memory