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: David Ahern <daahern@cisco.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
