IB/mlx5: Remove dead code

IB/mlx5: Remove dead code

mlx5_ib_dc_atomic_is_supported function is not used anywhere. Remove the
dead code.

Fixes: a60109dc9a95 ("IB/mlx5: Add support for extended atomic operations")
Link: https://lore.kernel.org/r/20191020064454.8551-1-leon@kernel.org
Signed-off-by: Ran Rozenstein <ranro@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>