igb: remove unused vmolr value

In an earlier version of igb_write_mc_addr_list() the vmolr register
was modified. This register is no longer accessed, although the variable
still exists. This patch removes it from the function.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>