e1000: Remove unused function e1000_mta_set.

Remove function e1000_mta_set, as it is no longer called

Signed-off-by: Dave Graham <david.graham@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>