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/modules/testing/test-instrumentation.nix

@@ -55,8 +55,7 @@ with import ../../lib/qemu-flags.nix { inherit pkgs; };
     systemd.services."serial-getty@hvc0".enable = false;
 
     # Only use a serial console, no TTY.
-    # hvc1: socket backdoor, see "Debugging NixOS tests" section in NixOS manual
-    virtualisation.qemu.consoles = [ "hvc1" qemuSerialDevice ];
+    virtualisation.qemu.consoles = [ qemuSerialDevice ];
 
     boot.initrd.preDeviceCommands =
       ''