config/releng: remove /etc/pacman.d/hooks/uncomment-mirrors.hook after it runs
This works around FS#49347.
Leaving the hook in the airootfs image will result in it being run when pacstrap
is run in the live environment. This should not happen as the hook is intended for the ISO build process only.
Fixes #91 (closed) introduced by !128 (merged).
Edited by nl6720