RDMA/mlx5: Delete struct mlx5_priv->mkey_table

No users are left, delete it.

Link: https://lore.kernel.org/r/20191009160934.3143-5-jgg@ziepe.ca


Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>