balloon: drop old stats code & API

balloon: drop old stats code & API

Next commit will re-enable balloon stats with a different interface, but
this old code conflicts with it. Let's drop it.

It's important to note that the QMP and HMP interfaces are also dropped
by this commit. That shouldn't be a problem though, because:

 1. All QMP fields are optional
 2. This feature has always been disabled

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>