igc: Clean up Rx timestamping logic

Differently from I210, I225 doesn't report Rx timestamps via the TS bit
Rx descriptor + RXSTMPL/RXSTMPH registers mechanism. Rx timestamps are
reported in the packet buffer only, which is implemented by igc_ptp_rx_
pktstamp(). So this patch removes igc_ptp_rx_rgtstamp() and all code
related to it, copied from igb driver.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>