Remove support for legacy live location

Remove code that was supposed to go away before the bullseye release.

Finalize the migration started in c6a17c7b.

Keeping the old path around breaks running update-initramfs on a live system, where backend/initramfs-tools/live.hook will copy libnss_dns.so.*, libnss_files.so.* from both /usr/lib and /usr/lib/live/mount/... into the initramfs.

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From old notes:

root@grml ~ # lsinitramfs /boot/initrd.img-5.6.0-2-amd64 | grep squash
usr/lib/live/mount/overlay/rw/var/tmp/mkinitramfs_pPceGY/usr/lib/live/mount/rootfs/grml64-full.squashfs
usr/lib/live/mount/overlay/rw/var/tmp/mkinitramfs_pPceGY/usr/lib/live/mount/rootfs/grml64-full.squashfs/lib
usr/lib/live/mount/overlay/rw/var/tmp/mkinitramfs_pPceGY/usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu
usr/lib/live/mount/overlay/rw/var/tmp/mkinitramfs_pPceGY/usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_dns-2.30.so
usr/lib/live/mount/overlay/rw/var/tmp/mkinitramfs_pPceGY/usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_dns.so.2
usr/lib/live/mount/overlay/rw/var/tmp/mkinitramfs_pPceGY/usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_files-2.30.so
usr/lib/live/mount/overlay/rw/var/tmp/mkinitramfs_pPceGY/usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_files.so.2
usr/lib/live/mount/rootfs/grml64-full.squashfs
usr/lib/live/mount/rootfs/grml64-full.squashfs/lib
usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu
usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_dns-2.30.so
usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_dns.so.2
usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_files-2.30.so
usr/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_files.so.2
usr/lib/modules/5.6.0-2-amd64/kernel/fs/squashfs
usr/lib/modules/5.6.0-2-amd64/kernel/fs/squashfs/squashfs.ko

root@grml ~ # find /lib /usr/lib -name 'libnss_dns.so.*' -o -name 'libnss_files.so.*'
/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_dns.so.2
/lib/live/mount/rootfs/grml64-full.squashfs/lib/x86_64-linux-gnu/libnss_files.so.2
/lib/x86_64-linux-gnu/libnss_dns.so.2
/lib/x86_64-linux-gnu/libnss_files.so.2