igc: Clean up the mac_info structure

collision_delta, tx_packet_delta, txcw, adaptive_ifs and
has_fwsm fields not in use.
This patch come to clean up the driver code.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>