Upgrading all packages broke initramfs

[jnschaeffer]

After running apt update and apt upgrade, nodes provisioned using raspberry-pi-netboot failed to boot cleanly. It looks like upgrading the system created a new initramfs at /boot/initramfs8 with a matching kernel image, which should work if auto_initramfs is set per official documentation, but /boot/config.txt was still attempting to boot from the old initrd file:

[all]
initramfs initrd.img-6.1.0-rpi8-rpi-v8 followkernel

Since the generated config file already has auto_initramfs set by default, the fix here is probably to just remove that initramfs config item.