Bluetooth: 6lowpan: remove unused function

Fix the following clang warning:

net/bluetooth/6lowpan.c:913:20: warning: unused function 'bdaddr_type'
[-Wunused-function].

net/bluetooth/6lowpan.c:106:35: warning: unused function
'peer_lookup_ba' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>