vdpa/mlx5: Don't clear mr struct on destroy MR

Clearing the mr struct erases the lock owner and causes warnings to be
emitted. It is not required to clear the mr so remove the memset call.

Fixes: 94abbccd

 ("vdpa/mlx5: Add shared memory registration code")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20230206121956.1149356-1-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>