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/programs/singularity.nix

@@ -7,8 +7,8 @@ in
 {
 
   options.programs.singularity = {
-    enable = mkEnableOption (mdDoc "singularity") // {
-      description = mdDoc ''
+    enable = mkEnableOption "singularity" // {
+      description = ''
         Whether to install Singularity/Apptainer with system-level overriding such as SUID support.
       '';
     };
@@ -18,7 +18,7 @@ in
     packageOverriden = mkOption {
       type = types.nullOr types.package;
       default = null;
-      description = mdDoc ''
+      description = ''
         This option provides access to the overridden result of `programs.singularity.package`.
 
         For example, the following configuration makes all the Nixpkgs packages use the overridden `singularity`:
@@ -43,7 +43,7 @@ in
       type = types.bool;
       default = true;
       example = false;
-      description = mdDoc ''
+      description = ''
         Whether to use top-level directories as LOCALSTATEDIR
         instead of the store path ones.
         This affects the SESSIONDIR of Apptainer/Singularity.
@@ -55,7 +55,7 @@ in
       type = types.bool;
       default = true;
       example = false;
-      description = mdDoc ''
+      description = ''
         Whether to enable the `--fakeroot` support of Singularity/Apptainer.
       '';
     };
@@ -68,7 +68,7 @@ in
       default = cfg.package.projectName != "apptainer";
       defaultText = literalExpression ''config.services.singularity.package.projectName != "apptainer"'';
       example = false;
-      description = mdDoc ''
+      description = ''
         Whether to enable the SUID support of Singularity/Apptainer.
       '';
     };