Monitor: Drop the print disabling mechanism

We can ignore calls to monitor_vprintf() in QMP mode and use
monitor_puts() directly in monitor_json_emitter().

This allows us to drop this ugly hack.

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