AngularJS 1.4.x - how to unregister a directive

var app = angular.module('plunker', []);

app.controller('MainCtrl', function($scope, fooDirective) {
  $scope.name = 'World';
  $scope.code = fooDirective;
});

app.directive('foo', function() {
  return function(scope, el, attr) {
    el.append('foo');
  };
});

//app.factory('fooDirective', () => angular.noop);

app.directive('foo', function() {
  return function(scope, el, attr) {
    el.append('foo2');
  };
});

<!DOCTYPE html>
<html ng-app="plunker">

  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link rel="stylesheet" href="style.css" />
    <script data-require="angular.js@1.4.x" src="https://ajax.googleapis.com/ajax/libs/angularjs/1.4.7/angular.js" data-semver="1.4.7"></script>
    <script src="app.js"></script>
  </head>

  <body ng-controller="MainCtrl">
    <p>Hello {{name}}!</p>
    <foo></foo>
    <pre>{{code|json}}</pre>
  </body>

</html>

/* Put your css in here */