appletalk: remove special handling code for ipddp

appletalk: remove special handling code for ipddp

After commit 1dab47139e61 ("appletalk: remove ipddp driver") removes the
config IPDDP, there is some minor code clean-up possible in the appletalk
network layer.

Remove some code in appletalk layer after the ipddp driver is gone.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231012063443.22368-1-lukas.bulwahn@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>