balloon: remove obsolete assert

I am currently being able to trigger this assert. It comes from the old days
before we found the opportunistic copying bug. Right now, it *is* possible to
have the balloons list to be empty, and still some old VMAs lying around.

Just remove the assertion.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>