igc: Remove forced_speed_duplex value

u8 forced_speed_duplex from value from igc_mac_info struct is not used.
This patch comes to tidy up the driver code.
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igc/igc_hw.h

@@ -89,8 +89,6 @@ struct igc_mac_info {
 	u32 mta_shadow[MAX_MTA_REG];
 	u16 rar_entry_count;
 
-	u8 forced_speed_duplex;
-
 	bool asf_firmware_present;
 	bool arc_subsystem_valid;