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/programs/command-not-found/command-not-found.nix

@@ -31,13 +31,12 @@ let
 
 in {
   options.programs.command-not-found = {
-    enable =
-      mkEnableOption (lib.mdDoc "command-not-found hook for interactive shell");
+    enable = mkEnableOption "command-not-found hook for interactive shell";
 
     dbPath = mkOption {
       default =
         "/nix/var/nix/profiles/per-user/root/channels/nixos/programs.sqlite";
-      description = lib.mdDoc ''
+      description = ''
         Absolute path to {file}`programs.sqlite`. By
         default this file will be provided by your channel
         (nixexprs.tar.xz).