Revert "ar71xx: Add GRO support to ag71xx"

This reverts commit 13e5e473699b92f171205e0f5c57c9ebe7922492.
This commit causes a severe regression in LAN->WAN routing performance
for several devices. This appears to be caused by the extra requirement
to validate the SKB checksum early in the rx path, which the ethernet
hardware does not do

Signed-off-by: Felix Fietkau <nbd@nbd.name>