vdpa_sim: don't select DMA_OPS

vdpa_sim has been fixed to not override the dma_map_ops in commit
6c3d329e ("vdpa_sim: get rid of DMA ops"), so don't select the
symbol and don't depend on HAS_DMA.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>