Remove session to allow `with_routing` to be called twice.

Fixes: https://github.com/rails/rails/issues/16814
@rafaelfranca Not in this case. If we call `reset!` here, it will build a new instance of `@integration_session` with `old_app`. Since an instance of `@integration_session` has already been built, the second call to `with_routing` will not build `@integration_session` with the new app and routes.