net/decnet: Delete obsolete TODO file

The TODO file here has not been updated from 2005, and the function
development described in the file have been implemented or abandoned.

Its existence will mislead developers seeking to view outdated information.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>