RDMA/mlx5: Delete unreachable handle_atomic code by simplifying SW completion

Handle atomic was left as unimplemented from 2013, remove the code till
this part will be developed.

Remove the dead code by simplifying SW completion logic which is supposed
to be the same for send and receive paths.

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au> # compile tested
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>