igb: Fix unused variable warning.

Commit eab6d18d "vlan: Don't check for vlan group before
vlan_tx_tag_present" removed the need for the adapter variable
in igb_xmit_frame_ring_adv().  This removes the variable as well
to avoid the compiler warning.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>