GUACAMOLE-724: Remove client-side "last used" timestamp (no longer needed).

The last-used timestamp was previously required to sort active clients
within the client panel in a sensible and deterministic order. This now
happens automatically via client groups, which are stored in an array
in order of last use. No code remains referencing the last-used
timestamp.

guacamole/src/main/frontend/src/app/client/controllers/clientController.js

@@ -460,12 +460,6 @@ angular.module('client').controller('clientController', ['$scope', '$routeParams
 
     });
 
-    // Update last used timestamp when the active client changes
-    $scope.$watch('client', function clientChanged(client) {
-        if (client)
-            client.lastUsed = new Date().getTime();
-    });
-
     // Update page icon when thumbnail changes
     $scope.$watch('client.thumbnail.canvas', function thumbnailChanged(canvas) {
         iconService.setIcons(canvas);

guacamole/src/main/frontend/src/app/client/types/ManagedClient.js

@@ -83,16 +83,6 @@ angular.module('client').factory('ManagedClient', ['$rootScope', '$injector',
          */
         this.id = template.id;
 
-        /**
-         * The time that the connection was last brought to the foreground of
-         * the current tab, as the number of milliseconds elapsed since
-         * midnight of January 1, 1970 UTC. If the connection has not yet been
-         * viewed, this will be 0.
-         *
-         * @type Number
-         */
-        this.lastUsed = template.lastUsed || 0;
-
         /**
          * The actual underlying Guacamole client.
          *