samples/subsys/mcumgr/smp_svr: disable log over shell

Mcumgr-cli application doesn't accept log on the communication
channel. Since #30370 was merged log messages are generated from
MCUBOOT_UTIL by default. That made `mcumgr image upload` failing.

Patch disables logging via shell which cure the issue.
It is not necessary if another application on the host intercepts
log messages (like picocom can).

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>