atl2: remove unused variable 'atl2_driver_string'

drivers/net/ethernet/atheros/atlx/atl2.c:40:19: warning: ‘atl2_driver_string’ defined but not used [-Wunused-const-variable=]
 static const char atl2_driver_string[] = "Atheros(R) L2 Ethernet Driver";
                   ^~~~~~~~~~~~~~~~~~

commit ea973742

 ("net/atheros: Clean atheros code from driver version")
left behind this, remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>