monitor: Use traditional command interface for HMP drive_del

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>