MAINTAINERS: Remove Jukka Rissanen as 6lowpan maintainer

I no longer work on this so better update the file.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Link: https://lore.kernel.org/r/20220527075625.9693-1-jukka.rissanen@linux.intel.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>