virtio_ring: remove flags check for unmap split indirect desc

When calling vring_unmap_one_split_indirect(), it will not encounter the
situation that the flags contains VRING_DESC_F_INDIRECT. So remove this
logic.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220224110402.108161-3-xuanzhuo@linux.alibaba.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>