trie/trie.go improve the delete performance

[Evolution404]

when nn is not nil, there is no need to check the number of non-nil entries

[holiman]

I find this hard to intuitively verify. I think it might be easier to follow if you break here, so replace this with

if nn != nil{	
	// n still contains at least two values and cannot be reduced.
	return true, n, nil
}

As far as I can tell, that^ change should be equivalent with your change. However, looking at it, I still don't see why this is necessarily true/correct?

[Evolution404]

if nn != nil, nn must be a child of n, which means that n itself doesn't need to be removed.

[Evolution404]

And before the deletion, there must be another child of n.