igc: Remove set but not used variable 'pci_using_dac'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/intel/igc/igc_main.c: In function 'igc_probe':
drivers/net/ethernet/intel/igc/igc_main.c:3535:11: warning:
 variable 'pci_using_dac' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit
d89f8841 ("igc: Add skeletal frame for Intel(R) 2.5G Ethernet Controller support")
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NSasha Neftin <sasha.neftin@intel.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igc/igc_main.c

@@ -3532,19 +3532,16 @@ static int igc_probe(struct pci_dev *pdev,
 	struct net_device *netdev;
 	struct igc_hw *hw;
 	const struct igc_info *ei = igc_info_tbl[ent->driver_data];
-	int err, pci_using_dac;
+	int err;
 
 	err = pci_enable_device_mem(pdev);
 	if (err)
 		return err;
 
-	pci_using_dac = 0;
 	err = dma_set_mask(&pdev->dev, DMA_BIT_MASK(64));
 	if (!err) {
 		err = dma_set_coherent_mask(&pdev->dev,
 					    DMA_BIT_MASK(64));
-		if (!err)
-			pci_using_dac = 1;
 	} else {
 		err = dma_set_mask(&pdev->dev, DMA_BIT_MASK(32));
 		if (err) {