nixos/rngd: Remove module entirely, leave an explaination

Per @shlevy's request on #96092.

nixos/modules/security/rngd.nix

@@ -1,56 +1,16 @@
-{ config, lib, pkgs, ... }:
-
-with lib;
-
+{ lib, ... }:
 let
-  cfg = config.security.rngd;
+  removed = k: lib.mkRemovedOptionModule [ "security" "rngd" k ];
 in
 {
-  options = {
-    security.rngd = {
-      enable = mkOption {
-        type = types.bool;
-        default = false;
-        description = ''
-          Whether to enable the rng daemon.  Devices that the kernel recognises
-          as entropy sources are handled automatically by krngd.
-        '';
-      };
-      debug = mkOption {
-        type = types.bool;
-        default = false;
-        description = "Whether to enable debug output (-d).";
-      };
-    };
-  };
-
-  config = mkIf cfg.enable {
-    systemd.services.rngd = {
-      bindsTo = [ "dev-random.device" ];
-
-      after = [ "dev-random.device" ];
-
-      # Clean shutdown without DefaultDependencies
-      conflicts = [ "shutdown.target" ];
-      before = [
-        "sysinit.target"
-        "shutdown.target"
-      ];
-
-      description = "Hardware RNG Entropy Gatherer Daemon";
-
-      # rngd may have to start early to avoid entropy starvation during boot with encrypted swap
-      unitConfig.DefaultDependencies = false;
-      serviceConfig = {
-        ExecStart = "${pkgs.rng-tools}/sbin/rngd -f"
-          + optionalString cfg.debug " -d";
-        # PrivateTmp would introduce a circular dependency if /tmp is on tmpfs and swap is encrypted,
-        # thus depending on rngd before swap, while swap depends on rngd to avoid entropy starvation.
-        NoNewPrivileges = true;
-        PrivateNetwork = true;
-        ProtectSystem = "full";
-        ProtectHome = true;
-      };
-    };
-  };
+  imports = [
+    (removed "enable" ''
+       rngd is not necessary for any device that the kernel recognises
+       as an hardware RNG, as it will automatically run the krngd task
+       to periodically collect random data from the device and mix it
+       into the kernel's RNG.
+    '')
+    (removed "debug"
+      "The rngd module was removed, so its debug option does nothing.")
+  ];
 }