igc: Fix registers definition

IGC_ICTXPTC and IGC_ICTXATC are already defined elsewhere, remove this
double definition. Also, remove unneeded registers as they are not
applicable to i225 devices.

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>