GetRequestablePieces: remove runtime correctness check

[veshij]

In profiles I collected correctness check

torrent/request-strategy/order.go

Lines 60 to 66 in c73c99b

	// Check that scan emits pieces in priority order.
	if lastItem.Ok {
	if _i.Less(&lastItem.Value) {
	panic("scan not in order")
	}
	}
	lastItem.Set(_i)

takes up to 10% cpu time. Would it make sense to remove it from runtime and add a unit-test for that instead (if not already)?

additional discussion is in #897 (comment)

[anacrolix]

I tried this on top of #897 (comment), and it knocked off another 10%. This can be merged independently of evaluating that other change. Thanks for the contribution!