RDMA/mlx5: Remove unused to_mibmr function

The to_mibmr function is unused, remove it.

Link: https://lore.kernel.org/r/20200705141143.47303-1-galpress@amazon.com


Signed-off-by: Gal Pressman <galpress@amazon.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>