ice: Remove rx_gro_dropped stat

Tracking of the rx_gro_dropped statistic was removed in
commit f73fc403 ("ice: drop dead code in ice_receive_skb()").
Remove the associated variables and its reporting to ethtool stats.

Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>