monitor: Remove unused password prompting fields

Commit 788cf9f8 removed the code for password prompting from the
monitor. Since then, the Monitor fields password_completion_cb and
password_opaque have been unused. Remove them.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20190613153405.24769-2-kwolf@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

monitor.c

@@ -222,8 +222,6 @@ struct Monitor {
 
     MonitorQMP qmp;
     gchar *mon_cpu_path;
-    BlockCompletionFunc *password_completion_cb;
-    void *password_opaque;
     mon_cmd_t *cmd_table;
     QTAILQ_ENTRY(Monitor) entry;