remove unused code (#2579)

fixes #2577

lib/utils.js

@@ -43,10 +43,6 @@
 
 "use strict";
 
-function slice(a, start) {
-    return Array.prototype.slice.call(a, start || 0);
-};
-
 function characters(str) {
     return str.split("");
 };
@@ -214,18 +210,6 @@ function mergeSort(array, cmp) {
     return _ms(array);
 };
 
-function set_difference(a, b) {
-    return a.filter(function(el){
-        return b.indexOf(el) < 0;
-    });
-};
-
-function set_intersection(a, b) {
-    return a.filter(function(el){
-        return b.indexOf(el) >= 0;
-    });
-};
-
 // this function is taken from Acorn [1], written by Marijn Haverbeke
 // [1] https://github.com/marijnh/acorn
 function makePredicate(words) {