Fix formatting for deprecation warning in Python 2.x

An extra comma in the deprecation warning print makes it print the warning as a tuple in Python 2.x:
```
$ celeryd
("\nThe 'celeryd' command is deprecated, please use 'celery worker' instead:\n\n$ celery worker\n\n",)
...
```
Removing the comma makes it print correctly on Py2 and Py3.