GUACAMOLE-55: Remove unneeded reference to document.

guacamole/src/main/webapp/app/clipboard/directives/guacClipboard.js

@@ -70,14 +70,6 @@ angular.module('clipboard').directive('guacClipboard', ['$injector',
         var $window          = $injector.get('$window');
         var clipboardService = $injector.get('clipboardService');
 
-        /**
-         * Reference to the window.document object.
-         *
-         * @private
-         * @type HTMLDocument
-         */
-        var document = $window.document;
-
         /**
          * Map of all currently pressed keys by keysym. If a particular key is
          * currently pressed, the value stored under that key's keysym within