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/virtualisation/docker-rootless.nix

@@ -18,7 +18,7 @@ in
     enable = mkOption {
       type = types.bool;
       default = false;
-      description = lib.mdDoc ''
+      description = ''
         This option enables docker in a rootless mode, a daemon that manages
         linux containers. To interact with the daemon, one needs to set
         {command}`DOCKER_HOST=unix://$XDG_RUNTIME_DIR/docker.sock`.
@@ -28,7 +28,7 @@ in
     setSocketVariable = mkOption {
       type = types.bool;
       default = false;
-      description = lib.mdDoc ''
+      description = ''
         Point {command}`DOCKER_HOST` to rootless Docker instance for
         normal users by default.
       '';
@@ -41,7 +41,7 @@ in
         ipv6 = true;
         "fixed-cidr-v6" = "fd00::/80";
       };
-      description = lib.mdDoc ''
+      description = ''
         Configuration for docker daemon. The attributes are serialized to JSON used as daemon.conf.
         See https://docs.docker.com/engine/reference/commandline/dockerd/#daemon-configuration-file
       '';