RDMA/mad: Remove snoop interface

Snoop interface is not used. Remove it.

Link: https://lore.kernel.org/r/20200413132408.931084-1-leon@kernel.org


Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>