mlx4_en: Remove remnants of LRO support

Commit fa37a958

 ('mlx4_en: Moving to
work with GRO') left behind the Kconfig depends/select, some dead
code and comments referring to LRO.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>