Simplify the license situation

There's no LICENSE file that mentions the Go authors in this repository, but since this code is covered by the CLA we can just move the name to the AUTHORS file and rely on the existing statement that references the Dart LICENSE file.

Change-Id: I3e8e04ef39e7ed6de0134cc33bf8a22e3d33389b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250792
Commit-Queue: Michael Thomsen <[email protected]>
Reviewed-by: Michael Thomsen <[email protected]>

Author: Hixie

AUTHORS

@@ -5,6 +5,7 @@
 
 Google Inc.
 The V8 project authors
+The Go Authors
 Arm Ltd. <*@arm.com>
 
 Ola Martin Bini <[email protected]>

sdk/lib/_internal/vm/lib/bigint_patch.dart

@@ -4,10 +4,6 @@
 
 // part of dart.core;
 
-// Copyright 2009 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.
-
 /*
  * Copyright (c) 2003-2005  Tom Wu
  * Copyright (c) 2012 Adam Singer ([email protected])