Remove deprecated APIs.

lib/src/web_socket/exception.dart

-// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/// This class is deprecated.
-///
-/// Use the [`web_socket_channel`][web_socket_channel] package instead.
-///
-/// [web_socket_channel]: https://pub.dartlang.org/packages/web_socket_channel
-@Deprecated("Will be removed in 3.0.0.")
-class CompatibleWebSocketException implements Exception {
-  final String message;
-
-  CompatibleWebSocketException([this.message]);
-
-  String toString() => message == null
-      ? "CompatibleWebSocketException" :
-        "CompatibleWebSocketException: $message";
-}