Monitor: Drop is_async_return()

If I understood it correcty, the is_async_return() logic was only
used to prevent QMP from issuing duplicated success responses
for asynchronous handlers.

However, QMP doesn't use do_info() anymore so this is dead logic
and (hopefully) can be safely dropped.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
