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: YueHaibing <yuehaibing@huawei.com>
Acked-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>