Remove instruction to block disk cache

By my understanding, unnecessary due to unmounting at the end of the instruction. Has lead to unstable and error-spewing ZFS setups, as discussed in this issue: https://github.com/NixOS/nixpkgs/issues/214871

docs/Getting Started/NixOS/Root on ZFS/3-system-configuration.rst

@@ -6,14 +6,6 @@ System Configuration
 .. contents:: Table of Contents
    :local:
 
-#. Disable cache, stale cache will prevent system from booting::
-
-     mkdir -p /mnt/etc/zfs/
-     rm -f /mnt/etc/zfs/zpool.cache
-     touch /mnt/etc/zfs/zpool.cache
-     chmod a-w /mnt/etc/zfs/zpool.cache
-     chattr +i /mnt/etc/zfs/zpool.cache
-
 #. Generate initial system configuration::
 
     nixos-generate-config --root /mnt