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/monitoring/opentelemetry-collector.nix

@@ -9,14 +9,14 @@ let
   settingsFormat = pkgs.formats.yaml {};
 in {
   options.services.opentelemetry-collector = {
-    enable = mkEnableOption (lib.mdDoc "Opentelemetry Collector");
+    enable = mkEnableOption "Opentelemetry Collector";
 
     package = mkPackageOption pkgs "opentelemetry-collector" { };
 
     settings = mkOption {
       type = settingsFormat.type;
       default = {};
-      description = lib.mdDoc ''
+      description = ''
         Specify the configuration for Opentelemetry Collector in Nix.
 
         See https://opentelemetry.io/docs/collector/configuration/ for available options.
@@ -26,7 +26,7 @@ in {
     configFile = mkOption {
       type = types.nullOr types.path;
       default = null;
-      description = lib.mdDoc ''
+      description = ''
         Specify a path to a configuration file that Opentelemetry Collector should use.
       '';
     };