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/manual.nix

@@ -16,7 +16,7 @@ in {
     manual.html.enable = mkOption {
       type = types.bool;
       default = false;
-      description = lib.mdDoc ''
+      description = ''
         Whether to install the HTML manual. This also installs the
         {command}`home-manager-help` tool, which opens a local
         copy of the Home Manager manual in the system web browser.
@@ -27,7 +27,7 @@ in {
       type = types.bool;
       default = true;
       example = false;
-      description = lib.mdDoc ''
+      description = ''
         Whether to install the configuration manual page. The manual can
         be reached by {command}`man home-configuration.nix`.
 
@@ -41,7 +41,7 @@ in {
       type = types.bool;
       default = false;
       example = true;
-      description = lib.mdDoc ''
+      description = ''
         Whether to install a JSON formatted list of all Home Manager
         options. This can be located at
         {file}`<profile directory>/share/doc/home-manager/options.json`,