monitor: Remove legacy "-mon default=on" parameter

The "default" parameter of the "-mon" option is useless since
QEMU v2.4.0, and marked as deprecated since QEMU v2.8.0. That
should have been long enough to let people update their scripts,
so time to remove it now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1513700253-10045-1-git-send-email-thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>