fix (provider): remove deprecated Experimental_LanguageModelV2Middleware (#5775)

Co-authored-by: Lars Grammel <lars.grammel@gmail.com>

Author: gr2m

.changeset/tricky-hats-fly.md

@@ -0,0 +1,5 @@
+---
+'@ai-sdk/provider': patch
+---
+
+Remove `Experimental_LanguageModelV2Middleware` type

packages/provider/src/language-model-middleware/v2/language-model-v2-middleware.ts

@@ -59,9 +59,3 @@ export type LanguageModelV2Middleware = {
     model: LanguageModelV2;
   }) => PromiseLike<Awaited<ReturnType<LanguageModelV2['doStream']>>>;
 };
-
-/**
- * @deprecated Use `LanguageModelV2Middleware` instead.
- */
-// TODO remove in v5
-export type Experimental_LanguageModelV2Middleware = LanguageModelV2Middleware;