net-gro: remove GRO_DROP

GRO_DROP can only be returned from napi_gro_frags()
if the skb has not been allocated by a prior napi_get_frags()

Since drivers must use napi_get_frags() and test its result
before populating the skb with metadata, we can safely remove
GRO_DROP since it offers no practical use.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>