blake2b: merge go125.go into blake2b_test.go

The go1.25 build constraint is guaranteed to always be satisfied because
the go directive is at 1.25.0, so the separated out go125.go file is not
needed. Move the assertion that the *xof type implements the hash.XOF
interface into a _test.go file to let it happen alongside other tests
in this package.

Change-Id: I65c886ede4d574a3168f28689f9529aa56586697
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/775781
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

blake2b/blake2b_test.go

@@ -14,6 +14,8 @@ import (
 	"testing"
 )
 
+var _ hash.XOF = (*xof)(nil)
+
 func fromHex(s string) []byte {
 	b, err := hex.DecodeString(s)
 	if err != nil {

blake2b/go125.go

@@ -1,11 +0,0 @@
-// Copyright 2025 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build go1.25
-
-package blake2b
-
-import "hash"
-
-var _ hash.XOF = (*xof)(nil)