nixos: remove all uses of lib.mdDoc

these changes were generated with nixq 0.0.2, by running

  nixq ">> lib.mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> Inherit >> mdDoc[remove]" --batchmode nixos/**.nix

two mentions of the mdDoc function remain in nixos/, both of which
are inside of comments.

Since lib.mdDoc is already defined as just id, this commit is a no-op as
far as Nix (and the built manual) is concerned.

nixos/modules/testing/service-runner.nix

@@ -107,7 +107,7 @@ let
   opts = { config, name, ... }: {
     options.runner = mkOption {
     internal = true;
-    description = lib.mdDoc ''
+    description = ''
         A script that runs the service outside of systemd,
         useful for testing or for using NixOS services outside
         of NixOS.

nixos/modules/testing/test-instrumentation.nix

@@ -56,14 +56,14 @@ in
 
   options.testing = {
 
-    initrdBackdoor = lib.mkEnableOption (lib.mdDoc ''
+    initrdBackdoor = lib.mkEnableOption ''
       enable backdoor.service in initrd. Requires
       boot.initrd.systemd.enable to be enabled. Boot will pause in
       stage 1 at initrd.target, and will listen for commands from the
       Machine python interface, just like stage 2 normally does. This
       enables commands to be sent to test and debug stage 1. Use
       machine.switch_root() to leave stage 1 and proceed to stage 2.
-    '');
+    '';
 
   };