QMP: Drop DEBUG event

This event has been introduced in the first round of QMP commits,
turns out that it's based on the usage of the EXCP_DEBUG macro,
which has discussable semantics when exposed through QMP.

As libvirt doesn't use this, let's just drop it.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
