IB/core: Removed unused function

rdma_addr_find_smac_by_sgid() is exported symbol not used by any kernel
module. Therefore its removed.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>