CODEOWNERS: remove myself as mcumgr/smp_udp owner

The mcumgr subsystem as a whole is already maintained by @nordicjm
(#57780) and I am no longer actively using mcumgr, so removing myself
from the equation and letting more capable hands take over.

Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@escolifesciences.com>