build: reduce ISO image size (don't keep APT indices on the live filesystem)

config/hooks/normal/0900-clear-apt-cache.hook.chroot

@@ -1,8 +1,5 @@
 #!/bin/sh
 echo "I: running $0"
 
-# clear APT package download cache to reduce the image size from ~100MB
-rm -vr /var/lib/apt/lists/
-
 # clear APT package list cache to reduce the image size from ~180MB
 rm -vr /var/cache/apt/