hw/virtio/vhost-user:Remove dead assignment in scrub_shadow_regions()

hw/virtio/vhost-user:Remove dead assignment in scrub_shadow_regions()

Clang static code analyzer show warning:
hw/virtio/vhost-user.c:606:9: warning: Value stored to 'mr' is never read
        mr = vhost_user_get_mr_data(reg->userspace_addr, &offset, &fd);
        ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20200827110311.164316-6-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>