Revert "nixos: installation: make installation ISOs USB bootable"

This reverts commit b7f6a138b13ca667464b31274a33124d252ebc8b.
Revert "nixos: iso-image: use syslinux bootloader for USB booting support"

This reverts commit 455db168b30d53c903c96be80ef843b79e0744ae.