settings: remove unstable apps key

it's not used anymore

dashboard/src/views/appstore.js

@@ -25,7 +25,6 @@ angular.module('Application').controller('AppStoreController', ['$scope', '$tran
     $scope.cachedCategory = ''; // used to cache the selected category while searching
     $scope.searchString = '';
     $scope.validSubscription = false;
-    $scope.unstableApps = false;
     $scope.subscription = {};
     $scope.memory = null; // { memory, swap }