nixos/eval-config: Remove NIXOS_EXTRA_MODULE_PATH

This removes the previously depricated NIXOS_EXTRA_MODULE_PATH
environment variable.

Tested with
$ nix-instantiate nixos/lib/eval-config.nix --arg modules '[{fileSystems."/".device="x";boot.loader.grub.enable=false;}]' -A config.system.build.toplevel
$ NIXOS_EXTRA_MODULE_PATH=$HOME/whatever.nix nix-instantiate nixos/lib/eval-config.nix --arg modules '[{fileSystems."/".device="x";boot.loader.grub.enable=false;}]' -A config.system.build.toplevel

nixos/doc/manual/release-notes/rl-2511.section.md

@@ -173,6 +173,20 @@
 
 - `renovate` was updated to v41. See the upstream release notes for [v40](https://github.com/renovatebot/renovate/releases/tag/40.0.0) and [v41](https://github.com/renovatebot/renovate/releases/tag/41.0.0) for breaking changes.
 
+- The "NIXOS_EXTRA_MODULE_PATH" variable from configuration evaluation has been deprecated.
+  We recommend a workflow where you update the expression files instead, but if you wish to continue
+  to use this variable, you may do so with a module like:
+
+  ```nix
+  {
+    imports = [
+      (builtins.getEnv "NIXOS_EXTRA_MODULE_PATH")
+    ];
+  }
+  ```
+
+  This has the benefit that your configuration hints at the non-standard workflow.
+
 - `i18n.inputMethod.fcitx5.plasma6Support` has been removed because qt6 is the only one used for fcitx5-configtool now.
 
 - The `boot.readOnlyNixStore` has been removed. Control over bind mount options on `/nix/store` is now offered by the `boot.nixStoreMountOpts` option.

nixos/lib/eval-config.nix

@@ -30,32 +30,7 @@ evalConfigArgs@{
   check ? true,
   prefix ? [ ],
   lib ? import ../../lib,
-  extraModules ?
+  extraModules ? [ ],
-    let
-      e = builtins.getEnv "NIXOS_EXTRA_MODULE_PATH";
-    in
-    lib.optional (e != "") (
-      lib.warn
-        ''
-          The NIXOS_EXTRA_MODULE_PATH environment variable is deprecated and will be
-          removed in NixOS 25.05.
-          We recommend a workflow where you update the expression files instead, but
-          if you wish to continue to use this variable, you may do so with a module like:
-
-          {
-            imports = [
-              (builtins.getEnv "NIXOS_EXTRA_MODULE_PATH")
-            ];
-          }
-
-          This has the benefit that your configuration hints at the
-          non-standard workflow.
-        ''
-        # NOTE: this import call is unnecessary and it even removes the file name
-        #       from error messages.
-        import
-        e
-    ),
 }:
 
 let