removing the advise to add virtualbox's guest additions in configuration.nix

... because `nixos-generate-config` currently understand it's running under virtualbox, and correctly adds the configuration in `/etc/nixos/hardware-configuration.nix`

Changed files
+1 -9
nixos
doc
manual
+1 -9
nixos/doc/manual/installation/installing-virtualbox-guest.xml
···
</orderedlist>
<para>
-
There are a few modifications you should make in configuration.nix. Enable
-
the virtualbox guest service in the main block:
-
</para>
-
-
<programlisting>
-
virtualisation.virtualbox.guest.enable = true;
-
</programlisting>
-
-
<para>
Enable booting:
</para>
···
</orderedlist>
<para>
+
There are a few modifications you should make in configuration.nix.
Enable booting:
</para>