Drop error result from Node.hashWithCount

[elias-orijtech]

Node.hashWithCount can only return errors in two cases:

iavl/node.go

Lines 336 to 343 in 64ffc27

	err := node.writeHashBytesRecursively(buf, version)
	if err != nil {
	return nil, err
	}
	_, err = h.Write(buf.Bytes())
	if err != nil {
	return nil, err
	}

The first case is its own error result, recursively. The second one is the error result from hash.Hash.Write which is documented to never return an error.

CC @odeke-em

[tac0turtle]

want to submit a pr?