Revert PR #5356. (#5492)

It doesn't really do what it says it does, and it introduces other
issues (single select elements re-open after selection).

Fixes #5490

src/js/select2/core.js

@@ -276,23 +276,6 @@ define([
 
   Select2.prototype._registerEvents = function () {
     var self = this;
-
-    this.on('focus', function () {
-      self.$container.addClass('select2-container--focus');
-
-      if (!self.$container.hasClass('select2-container--disabled') &&
-          !self.isOpen()) {
-        if (self.options.get('multiple')) {
-          window.setTimeout(function () {
-            self.open();
-          },
-          self.options.get('ajax') ? 300 : 100);
-        }
-        else {
-          self.open();
-        }
-      }
-    });
 
     this.on('open', function () {
       self.$container.addClass('select2-container--open');