treewide: remove now-redundant lib.mdDoc calls

These (and the `*MD` functions apart from `literalMD`) are now no-ops
in nixpkgs and serve no purpose other than to add additional noise and
potentially mislead people into thinking unmarked DocBook documentation
will still be accepted.

Note that if backporting changes including documentation to 23.05,
the `mdDoc` calls will need to be re-added.

To reproduce this commit, run:

    $ NIX_PATH=nixpkgs=flake:nixpkgs/e7e69199f0372364a6106a1e735f68604f4c5a25 \
      nix shell nixpkgs#coreutils \
      -c find . -name '*.nix' \
      -exec nix run -- github:emilazy/nix-doc-munge/98dadf1f77351c2ba5dcb709a2a171d655f15099 \
      --strip {} +
    $ ./format

modules/i18n/input-method/default.nix

@@ -31,7 +31,7 @@ in {
           (types.enum [ "fcitx" "fcitx5" "nabi" "uim" "hime" "kime" ]);
         default = null;
         example = "fcitx5";
-        description = lib.mdDoc ''
+        description = ''
           Select the enabled input method. Input methods are software to input
           symbols that are not available on standard input devices.
 
@@ -65,7 +65,7 @@ in {
         internal = true;
         type = types.nullOr types.path;
         default = null;
-        description = lib.mdDoc ''
+        description = ''
           The input method method package.
         '';
       };

modules/i18n/input-method/fcitx5.nix

@@ -13,7 +13,7 @@ in {
         type = with types; listOf package;
         default = [ ];
         example = literalExpression "with pkgs; [ fcitx5-rime ]";
-        description = lib.mdDoc ''
+        description = ''
           Enabled Fcitx5 addons.
         '';
       };

modules/i18n/input-method/kime.nix

@@ -26,7 +26,7 @@ in {
             };
           }
         '';
-        description = lib.mdDoc ''
+        description = ''
           kime configuration. Refer to
           <https://github.com/Riey/kime/blob/develop/docs/CONFIGURATION.md>
           for details on supported values.

modules/i18n/input-method/uim.nix

@@ -11,7 +11,7 @@ in {
         type = types.enum [ "gtk" "gtk3" "gtk-systray" "gtk3-systray" "qt4" ];
         default = "gtk";
         example = "gtk-systray";
-        description = lib.mdDoc ''
+        description = ''
           Selected UIM toolbar.
         '';
       };