Plugins: Fix Default Nav URL for dashboard includes (#47143)

* Plugins: Fix Default Nav URL for dashboard includes

* update nav links

* PR comments

pkg/plugins/manager/testdata/test-app-with-includes/plugin.json

@@ -20,8 +20,7 @@
     {
       "type": "dashboard",
       "name": "Nginx Memory",
-      "path": "dashboards/memory.json",
-      "defaultNav": true
+      "path": "dashboards/memory.json"
     },
     {
       "type": "page",