Revert "Add ssh backdoor to VM tests infrastructure."

This reverts commit d6e3db44cf.

See #53935 for explanations. In short, it may be causing issues with
tests on the build infrastructure.

nixos/doc/manual/development/debugging-nixos-tests.xml

@@ -1,37 +0,0 @@
-<section xmlns="http://docbook.org/ns/docbook"
-        xmlns:xlink="http://www.w3.org/1999/xlink"
-        xmlns:xi="http://www.w3.org/2001/XInclude"
-        version="5.0"
-        xml:id="sec-debugging-nixos-tests">
- <title>Debugging NixOS tests</title>
-
- <para>
-  Tests may fail and infrastructure offers access to inspect machine state.
- </para>
-
- <para>
-  To prevent test from stopping and cleaning up, insert a sleep command:
- </para>
-
-<programlisting>
-$machine->succeed("sleep 84000");
-</programlisting>
-
- <para>
-  As soon as machine starts run as root:
- </para>
-
-<programlisting>
-nix-shell -p socat --run "socat STDIO,raw,echo=0,escape=0x11 UNIX:/tmp/nix-build-vm-test-run-*.drv-0/vm-state-machine/backdoor"
-</programlisting>
-
- <para>
-  You may need to find the correct path, replacing <literal>/tmp</literal>,
-  <literal>*</literal> or <literal>machine</literal>.
- </para>
-
- <para>
-  Press "enter" to open up console and login as "root". After you're done,
-  press "ctrl-q" to exit the console.
- </para>
-</section>

nixos/doc/manual/development/nixos-tests.xml

@@ -16,5 +16,4 @@ xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/tests">nixos/test
  <xi:include href="writing-nixos-tests.xml" />
  <xi:include href="running-nixos-tests.xml" />
  <xi:include href="running-nixos-tests-interactively.xml" />
- <xi:include href="debugging-nixos-tests.xml" />
 </chapter>