monitor: Use traditional command interface for HMP drive_del

All QMP commands use the "new" handler interface (mhandler.cmd_new).
Most HMP commands still use the traditional interface (mhandler.cmd),
but a few use the "new" one.  Complicates handle_user_command() for no
gain, so I'm converting these to the traditional interface.

For drive_del, that's easy: hmp_drive_del() sheds its unused last
parameter, and its return value, which the caller ignored anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/include/sysemu/blockdev.h b/include/sysemu/blockdev.h
index 7ca59b5..3104150 100644
--- a/include/sysemu/blockdev.h
+++ b/include/sysemu/blockdev.h
@@ -66,5 +66,5 @@
 void qmp_change_blockdev(const char *device, const char *filename,
                          const char *format, Error **errp);
 void hmp_commit(Monitor *mon, const QDict *qdict);
-int hmp_drive_del(Monitor *mon, const QDict *qdict, QObject **ret_data);
+void hmp_drive_del(Monitor *mon, const QDict *qdict);
 #endif