IB/mlx5: Remove set but not used variable

Remove 'del_mkey' variable that is set but not used.

Fixes: 534fd7aa ("IB/mlx5: Manage indirection mkey upon DEVX flow for ODP")
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>