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/continuous-integration/gitea-actions-runner.nix

@@ -60,17 +60,17 @@ in
 
     instances = mkOption {
       default = {};
-      description = lib.mdDoc ''
+      description = ''
         Gitea Actions Runner instances.
       '';
       type = attrsOf (submodule {
         options = {
-          enable = mkEnableOption (lib.mdDoc "Gitea Actions Runner instance");
+          enable = mkEnableOption "Gitea Actions Runner instance";
 
           name = mkOption {
             type = str;
             example = literalExpression "config.networking.hostName";
-            description = lib.mdDoc ''
+            description = ''
               The name identifying the runner instance towards the Gitea/Forgejo instance.
             '';
           };
@@ -78,7 +78,7 @@ in
           url = mkOption {
             type = str;
             example = "https://forge.example.com";
-            description = lib.mdDoc ''
+            description = ''
               Base URL of your Gitea/Forgejo instance.
             '';
           };
@@ -86,7 +86,7 @@ in
           token = mkOption {
             type = nullOr str;
             default = null;
-            description = lib.mdDoc ''
+            description = ''
               Plain token to register at the configured Gitea/Forgejo instance.
             '';
           };
@@ -94,7 +94,7 @@ in
           tokenFile = mkOption {
             type = nullOr (either str path);
             default = null;
-            description = lib.mdDoc ''
+            description = ''
               Path to an environment file, containing the `TOKEN` environment
               variable, that holds a token to register at the configured
               Gitea/Forgejo instance.
@@ -113,7 +113,7 @@ in
                 #"native:host"
               ]
             '';
-            description = lib.mdDoc ''
+            description = ''
               Labels used to map jobs to their runtime environment. Changing these
               labels currently requires a new registration token.
 
@@ -122,7 +122,7 @@ in
             '';
           };
           settings = mkOption {
-            description = lib.mdDoc ''
+            description = ''
               Configuration for `act_runner daemon`.
               See https://gitea.com/gitea/act_runner/src/branch/main/internal/pkg/config/config.example.yaml for an example configuration
             '';
@@ -158,7 +158,7 @@ in
                 wget
               ]
             '';
-            description = lib.mdDoc ''
+            description = ''
               List of packages, that are available to actions, when the runner is configured
               with a host execution label.
             '';