configs/releng: move the mirror uncommenting sed command from customize_airootfs.sh to a pacman hook

After pacman-mirrorlist is installed, /etc/pacman.d/hooks/uncomment-mirrors.hook will run a sed command which uncomments all Server lines in /etc/pacman.d/mirrorlist.
This brings us another step closer to the complete removal of customize_airootfs.sh.

Related to archlinux/archiso#21 .