lib/types, lib/modules: Remove unused extensionOffset

lib/modules.nix

@@ -113,10 +113,6 @@ rec {
                   args ? {}
                 , # This would be remove in the future, Prefer _module.check option instead.
                   check ? true
-                  # Internal variable to avoid `_key` collisions regardless
-                  # of `extendModules`. Used in `submoduleWith`.
-                  # Test case: lib/tests/modules, "168767"
-                , extensionOffset ? 0
                 }:
     let
       withWarnings = x:
@@ -345,17 +341,15 @@ rec {
         modules ? [],
         specialArgs ? {},
         prefix ? [],
-        extensionOffset ? length modules,
         }:
           evalModules (evalModulesArgs // {
             modules = regularModules ++ modules;
             specialArgs = evalModulesArgs.specialArgs or {} // specialArgs;
             prefix = extendArgs.prefix or evalModulesArgs.prefix or [];
-            inherit extensionOffset;
           });
 
       type = lib.types.submoduleWith {
-        inherit modules specialArgs extensionOffset;
+        inherit modules specialArgs;
       };
 
       result = withWarnings {

lib/types.nix

@@ -571,11 +571,6 @@ rec {
       , specialArgs ? {}
       , shorthandOnlyDefinesConfig ? false
       , description ? null
-
-        # Internal variable to avoid `_key` collisions regardless
-        # of `extendModules`. Wired through by `evalModules`.
-        # Test case: lib/tests/modules, "168767"
-      , extensionOffset ? 0
       }@attrs:
       let
         inherit (lib.modules) evalModules;
@@ -623,7 +618,6 @@ rec {
           (base.extendModules {
             modules = [ { _module.args.name = last loc; } ] ++ allModules defs;
             prefix = loc;
-            extensionOffset = extensionOffset + length defs;
           }).config;
         emptyValue = { value = {}; };
         getSubOptions = prefix: (base.extendModules