kitchen: Re-enable graceful shutdown timeout

Version 0.5.1 fixes the issue with `uvicorn` hanging on shutdown because
of the WebSocket message queue.

kitchen/kitchen.yaml

@@ -35,7 +35,6 @@ spec:
         image: git.pyrocufflink.net/packages/kitchen:master
         args:
         - --host=0.0.0.0
-        - --timeout-graceful-shutdown=0
         env:
         - name: TZ
           value: America/Chicago