frontend-plugin-api: default plugin routes to empty

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

plugins/techdocs/alpha-api-report.md

@@ -3,7 +3,6 @@
 > Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
 
 ```ts
-import { AnyExternalRoutes } from '@backstage/frontend-plugin-api';
 import { BackstagePlugin } from '@backstage/frontend-plugin-api';
 import { Extension } from '@backstage/frontend-plugin-api';
 import { RouteRef } from '@backstage/frontend-plugin-api';
@@ -18,7 +17,7 @@ const _default: BackstagePlugin<
       namespace: string;
     }>;
   },
-  AnyExternalRoutes
+  {}
 >;
 export default _default;