function clean( hash, threshold ) {
	var node = hash.first();
	while ( node ) {
		if ( node.value[ 1 ] < threshold ) {
			hash.remove( node );
		}
		node = node.next;
	}
	return hash;
}
Cleans a hash by removing nodes having counts below a provided threshold.

@private
@param {LinkedList} hash - input hash
@param {PositiveInteger} threshold - count threshold
@returns {LinkedList} input hash