wireguard: receive: account for napi_gro_receive never returning GRO_DROP

The napi_gro_receive function no longer returns GRO_DROP ever, making
handling GRO_DROP dead code. This commit removes that dead code.
Further, it's not even clear that device drivers have any business in
taking action after passing off received packets; that's arguably out of
their hands.

Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Fixes: 6570bc79 ("net: core: use listified Rx for GRO_NORMAL in napi_gro_receive()")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>