modem_cellular: lara_r6: disable modem's own LWM2M client

U-blox LARA-R6 LWM2M client is enabled by default. Not only causes this
the modem to connect to U-blox's server on its own, it also for some
reason causes the modem to reply "Destination unreachable" to DNS answers
from DNS requests that we send

Signed-off-by: Emil Lindqvist <emil@lindq.gr>