Shell: Remove `sleep` command

The sleep command is bundled with CRaSH, removing it from the jar packaging.

Fixes #24.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>