appletalk: remove "config IPDDP_DECAP"

The Kconfig symbol IPDDP_DECAP got added in v2.1.75. It has never been
used. Its entry can safely be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>