fix(@nguniversal/express-engine): remove default value of appDir option

This option is no longer used by the universal schematic.

Fixes #2779

Author: alan-agius4

modules/express-engine/schematics/install/schema.json

@@ -37,7 +37,7 @@
       "type": "string",
       "format": "path",
       "description": "The name of the application directory.",
-      "default": "app"
+      "x-deprecated": "This option has no effect."
     },
     "rootModuleFileName": {
       "type": "string",

modules/express-engine/schematics/install/schema.ts

@@ -29,6 +29,7 @@ export interface Schema {
   serverPort?: number;
   /**
    * The name of the application directory.
+   * @deprecated This option has no longer any effect
    */
   appDir?: string;
   /**