remove bind polyfill from single_file_diff.js

app/assets/javascripts/single_file_diff.js

 /* eslint-disable func-names, prefer-arrow-callback, space-before-function-paren, no-var, prefer-rest-params, wrap-iife, one-var, one-var-declaration-per-line, consistent-return, no-param-reassign, max-len */
 (function() {
-  var bind = function(fn, me) { return function() { return fn.apply(me, arguments); }; };
-
   window.SingleFileDiff = (function() {
     var COLLAPSED_HTML, ERROR_HTML, LOADING_HTML, WRAPPER;
@@ -16,7 +14,7 @@
     function SingleFileDiff(file) {
       this.file = file;
-      this.toggleDiff = bind(this.toggleDiff, this);
+      this.toggleDiff = this.toggleDiff.bind(this);
       this.content = $('.diff-content', this.file);
       this.$toggleIcon = $('.diff-toggle-caret', this.file);
       this.diffForPath = this.content.find('[data-diff-for-path]').data('diff-for-path');