configs/releng: move locale-gen from customize_airootfs.sh to a pacman hook

This finally removes customize_airootfs.sh from releng.

Fixes archlinux/archiso#21 .

configs/releng/airootfs/etc/pacman.d/hooks/40-locale-gen.hook

+# remove from airootfs!
+[Trigger]
+Operation = Install
+Type = Package
+Target = glibc
+
+[Action]
+Description = Uncommenting en_US.UTF-8 locale and running locale-gen...
+When = PostTransaction
+Depends = glibc
+Depends = sed
+Depends = sh
+Exec = /bin/sh -c "sed -i 's/#\(en_US\.UTF-8\)/\1/' /etc/locale.gen && locale-gen"

configs/releng/airootfs/root/customize_airootfs.sh

-#!/usr/bin/env bash
-#
-# SPDX-License-Identifier: GPL-3.0-or-later
-
-set -e -u
-
-# Warning: customize_airootfs.sh is deprecated! Support for it will be removed in a future archiso version.
-
-sed -i 's/#\(en_US\.UTF-8\)/\1/' /etc/locale.gen
-locale-gen