Drop support for non-atomic non-mutex platforms.

src/ports/SkMutex_none.h

-/*
- * Copyright 2013 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#ifndef SkMutex_none_DEFINED
-#define SkMutex_none_DEFINED
-
-/** Non-mutex mutex for uniprocessor systems. */
-
-struct SkBaseMutex {
-    void acquire() { }
-    void release() { }
-};
-
-class SkMutex : public SkBaseMutex {
-public:
-    SkMutex() { }
-    ~SkMutex() { }
-
-private:
-    SkMutex(const SkMutex&);
-    SkMutex& operator=(const SkMutex&);
-};
-
-// Using POD-style initialization prevents the generation of a static initializer.
-#define SK_DECLARE_STATIC_MUTEX(name) static SkBaseMutex name = { }
-
-// Special case used when the static mutex must be available globally.
-#define SK_DECLARE_GLOBAL_MUTEX(name) SkBaseMutex name = { }
-
-#endif