refactor(@schematics/angular): use multiselect auto-discovery in guard schematic

Author: clydin

packages/schematics/angular/guard/schema.json

@@ -54,6 +54,7 @@
       "type": "array",
       "description": "Specifies which interfaces to implement.",
       "uniqueItems": true,
+      "minItems": 1,
       "items": {
         "enum": [
           "CanActivate",
@@ -65,17 +66,7 @@
       "default": [
         "CanActivate"
       ],
-      "minItems": 1,
-      "x-prompt": {
-        "message": "Which interfaces would you like to implement?",
-        "type": "list",
-        "multiselect": true,
-        "items": [
-          "CanActivate",
-          "CanActivateChild",
-          "CanLoad"
-        ]
-      }
+      "x-prompt": "Which interfaces would you like to implement?"
     }
   },
   "required": [