configs/releng/airootfs/etc/mkinitcpio.conf: remove archiso_shutdown

The archiso_shutdown hook has been obsolete since mkinitcpio 16.
https://lists.archlinux.org/pipermail/arch-dev-public/2013-December/025742.html

Related to mkinitcpio/mkinitcpio-archiso#8.
7 jobs for rm-archiso_shutdown in 36 minutes and 4 seconds (queued for 4 seconds)