Revert "[reboot] stop docker service before rebooting (sonic-net#423)" (sonic-net#434)

This reverts commit 220e269.

Author: yxieca

scripts/reboot

@@ -15,10 +15,6 @@ function stop_sonic_services()
         docker exec -i syncd /usr/bin/syncd_request_shutdown --cold > /dev/null
         sleep 3
     fi
-
-    # Stop docker service to avoid 5-10 minutes delay in shutdown
-    # while waiting for existing docker containers to quit.
-    timeout 30s systemctl stop docker
 }
 
 function clear_warm_boot()