samples: net: cellular_modem: disable AT shell for NRF91_SLM

The NRF91_SLM has no additional AT channel for the AT shell to
use. This commit disables the AT shell for boards which use
the NRF91_SLM.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>