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/services/networking/epmd.nix

@@ -11,7 +11,7 @@ in
     enable = mkOption {
       type = types.bool;
       default = false;
-      description = lib.mdDoc ''
+      description = ''
         Whether to enable socket activation for Erlang Port Mapper Daemon (epmd),
         which acts as a name server on all hosts involved in distributed
         Erlang computations.
@@ -22,7 +22,7 @@ in
       {
         type = types.str;
         default = "[::]:4369";
-        description = lib.mdDoc ''
+        description = ''
           the listenStream used by the systemd socket.
           see https://www.freedesktop.org/software/systemd/man/systemd.socket.html#ListenStream= for more information.
           use this to change the port epmd will run on.