We introduced a RAII object (ScopedSetRouterService) to temporarily switch the Service to a RouterService associated with the opCtx when initializing the UserCacheInvalidator.

This was originally done to initialize the UserCacheInvalidator for MongoD instances acting as routers. Since this is no longer the case (see SERVER-105113), we no longer need to change the service. The only remaining usage is in MongoS, where the Service is already a RouterService.

Let's remove this dead code.