ieee802154: remove nl802154 unused functions

The include/net/nl802154.h file contains a lot of prototypes which are
not used inside of ieee802154 subsystem. This patch removes this file
and make the only one used prototype "ieee802154_nl_start_confirm" as
static declaration in ieee802154/nl-mac.c

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>