RDMA/mlx5: Remove redundant destroy QP call

After major refactoring in create QP flow, it is no needed to call
to destroy QP in XRC_TGT flow.

Link: https://lore.kernel.org/r/20200427154636.381474-35-leon@kernel.org


Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>