kconfig: Removed source "[...]mgmt/mcumgr/mgmt/port/zephyr/Kconfig"

This Kconfig 'source' statement had no effect as it was referencing a
non-existing Kconfig file. It is not clear if the intention is to
include-if-exists, but I presume not.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>