Revert "allow out-of-tree nixos modules"

This reverts commit b609ff4fcf.

It turns out this can just be done using "require".

default.nix

@@ -1,15 +1,12 @@
 { configuration ? import ./lib/from-env.nix "NIXOS_CONFIG" <nixos-config>
-, extraModulesPath ? builtins.getEnv "NIXOS_EXTRA_MODULES" 
 , system ? builtins.currentSystem
 }:
 
 let
 
-  extraModules = if extraModulesPath == "" then [] else import extraModulesPath;
-
   eval = import ./lib/eval-config.nix {
     inherit system;
-    modules = [ configuration ] ++ extraModules;
+    modules = [ configuration ];
   };
 
   inherit (eval) config pkgs;
@@ -17,7 +14,7 @@ let
   # This is for `nixos-rebuild build-vm'.
   vmConfig = (import ./lib/eval-config.nix {
     inherit system;
-    modules = [ configuration ./modules/virtualisation/qemu-vm.nix ] ++ extraModules;
+    modules = [ configuration ./modules/virtualisation/qemu-vm.nix ];
   }).config;
 
   # This is for `nixos-rebuild build-vm-with-bootloader'.
@@ -27,7 +24,7 @@ let
       [ configuration
         ./modules/virtualisation/qemu-vm.nix
         { virtualisation.useBootLoader = true; }
-      ] ++ extraModules;
+      ];
   }).config;
 
 in

modules/security/sudo.nix

@@ -35,7 +35,6 @@ in
           # Environment variables to keep for root and %wheel.
           Defaults:root,%wheel env_keep+=LOCALE_ARCHIVE
           Defaults:root,%wheel env_keep+=NIX_PATH
-          Defaults:root,%wheel env_keep+=NIXOS_EXTRA_MODULES
           Defaults:root,%wheel env_keep+=TERMINFO_DIRS
 
           # Keep SSH_AUTH_SOCK so that pam_ssh_agent_auth.so can do its magic.