vhost: remove an incorrect assert

The 'to' can go negative when the first region gets removed
(it gets incremented by to 0 immediately afterward), which
makes the assertion fail. Nothing breaks if
to < 0 here so just remove the assert.
Tested-by: NDavid Ahern <daahern@cisco.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/vhost.c

@@ -120,7 +120,6 @@ static void vhost_dev_unassign_memory(struct vhost_dev *dev,
         if (start_addr <= reg->guest_phys_addr && memlast >= reglast) {
             --dev->mem->nregions;
             --to;
-            assert(to >= 0);
             ++overlap_middle;
             continue;
         }