doc: installing-usb: removes notes about unetbootin.

They are known to cause more issues than solving issues; futhermore
using `dd` should work everywhere without fail.

(cherry picked from commit 8467dc8)

nixos/doc/manual/installation/installing-usb.xml

@@ -36,34 +36,5 @@ $ sudo dd bs=1m if=nix.iso of=/dev/rdiskN
  <para>
   The <command>dd</command> utility will write the image verbatim to the drive,
   making it the recommended option for both UEFI and non-UEFI installations.
-  For non-UEFI installations, you can alternatively use
-  <link xlink:href="http://unetbootin.sourceforge.net/">unetbootin</link>. If
-  you cannot use <command>dd</command> for a UEFI installation, you can also
-  mount the ISO, copy its contents verbatim to your drive, then either:
-  <itemizedlist>
-   <listitem>
-    <para>
-     Change the label of the disk partition to the label of the ISO (visible
-     with the blkid command), or
-    </para>
-   </listitem>
-   <listitem>
-    <para>
-     Edit <filename>loader/entries/nixos-livecd.conf</filename> on the drive
-     and change the <literal>root=</literal> field in the
-     <literal>options</literal> line to point to your drive (see the
-     documentation on <literal>root=</literal> in
-     <link xlink:href="https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt">
-     the kernel documentation</link> for more details).
-    </para>
-   </listitem>
-   <listitem>
-    <para>
-     If you want to load the contents of the ISO to ram after bootin (So you
-     can remove the stick after bootup) you can append the parameter
-     <literal>copytoram</literal> to the <literal>options</literal> field.
-    </para>
-   </listitem>
-  </itemizedlist>
  </para>
 </section>