tuntap: remove unused variable in __tun_detach()

The variable dev is initialized but never used
otherwise, so remove the unused variable.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>