Removed stubs/singleton.h

This header is no longer used, so we can delete it. I also tweaked
Makefile.am to fix the distcheck test.

Makefile.am

@@ -209,6 +209,7 @@ csharp_EXTRA_DIST=                                                           \
 
 java_EXTRA_DIST=                                                                   \
   java/README.md                                                                   \
+  java/bom/pom.xml                                                                 \
   java/core/generate-sources-build.xml                                             \
   java/core/generate-test-sources-build.xml                                        \
   java/core/pom.xml                                                                \

src/google/protobuf/stubs/singleton.h

-// Protocol Buffers - Google's data interchange format
-// Copyright 2014 Google Inc.  All rights reserved.
-// https://developers.google.com/protocol-buffers/
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are
-// met:
-//
-//     * Redistributions of source code must retain the above copyright
-// notice, this list of conditions and the following disclaimer.
-//     * Redistributions in binary form must reproduce the above
-// copyright notice, this list of conditions and the following disclaimer
-// in the documentation and/or other materials provided with the
-// distribution.
-//     * Neither the name of Google Inc. nor the names of its
-// contributors may be used to endorse or promote products derived from
-// this software without specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-#ifndef GOOGLE_PROTOBUF_STUBS_SINGLETON_H__
-#define GOOGLE_PROTOBUF_STUBS_SINGLETON_H__
-
-#include <google/protobuf/stubs/common.h>
-#include <google/protobuf/stubs/once.h>
-
-namespace google {
-namespace protobuf {
-namespace internal {
-template<typename T>
-class Singleton {
- public:
-  static T* get() {
-    GoogleOnceInit(&once_, &Singleton<T>::Init);
-    return instance_;
-  }
-  static void ShutDown() {
-    delete instance_;
-    instance_ = nullptr;
-  }
- private:
-  static void Init() {
-    instance_ = new T();
-  }
-  static ProtobufOnceType once_;
-  static T* instance_;
-};
-
-template<typename T>
-ProtobufOnceType Singleton<T>::once_;
-
-template<typename T>
-T* Singleton<T>::instance_ = nullptr;
-}  // namespace internal
-}  // namespace protobuf
-}  // namespace google
-
-#endif  // GOOGLE_PROTOBUF_STUBS_SINGLETON_H__