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>

drivers/net/igb/igb_main.c

@@ -4107,7 +4107,6 @@ static inline int igb_maybe_stop_tx(struct igb_ring *tx_ring, int size)
 netdev_tx_t igb_xmit_frame_ring_adv(struct sk_buff *skb,
 				    struct igb_ring *tx_ring)
 {
-	struct igb_adapter *adapter = netdev_priv(tx_ring->netdev);
 	int tso = 0, count;
 	u32 tx_flags = 0;
 	u16 first;