Clean up dead xfermode opts code.

src/opts/SkXfermode_opts_SSE2.h

-/*
- * Copyright 2014 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#ifndef SkXfermode_opts_SSE2_DEFINED
-#define SkXfermode_opts_SSE2_DEFINED
-
-#include "SkTypes.h"
-#include "SkXfermode_proccoeff.h"
-
-class SK_API SkSSE2ProcCoeffXfermode : public SkProcCoeffXfermode {
-public:
-    SkSSE2ProcCoeffXfermode(const ProcCoeff& rec, SkXfermode::Mode mode,
-                            void* procSIMD)
-        : INHERITED(rec, mode), fProcSIMD(procSIMD) {}
-
-    void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
-                const SkAlpha aa[]) const override;
-    void xfer16(uint16_t dst[], const SkPMColor src[],
-                int count, const SkAlpha aa[]) const override;
-
-    SK_TO_STRING_OVERRIDE()
-
-private:
-    void* fProcSIMD;
-    typedef SkProcCoeffXfermode INHERITED;
-};
-
-SkProcCoeffXfermode* SkPlatformXfermodeFactory_impl_SSE2(const ProcCoeff& rec,
-                                                         SkXfermode::Mode mode);
-
-#endif // SkXfermode_opts_SSE2_DEFINED