Remove default promgen command for docker entrypoint

By having a default value here, it makes it more difficult if you want
to run a one-off container for testing. Removing the default makes it
easier if one wants to run /bin/sh or any other command. The remaining
entries are just shortcuts for celery/gunicorn.

docker/docker-entrypoint.sh

@@ -36,10 +36,6 @@ web)
   shift
   set -- gunicorn "promgen.wsgi:application" "$@"
   ;;
-*)
-  # Shortcuts for some commonly used django commands
-  set -- promgen "$@"
-  ;;
 esac
 
 # Finally exec our command