Why are forgetting and sleep relevant? If someone invented a pill that gave you a perfect memory and removed the need to sleep, would you stop being generally intelligent if you took it?
There's a sweet spot between knowing enough and knowing little enough so that you get the right answer and get it quickly enough.
Correlation != Causation. While they very likely might be relevant, I've not seen anything to conclusively prove that it is. The ability to forget is important to humans because we are emotional beings, but I don't think that necessarily is a requirement for generalized intelligence. "sleep" (as in what happens during sleep, not the act specifically itself) on the other hand is very likely important, but again, not proven.