Revert "Revert "remove (empty) SkXfermode.h""

This reverts commit c5d0147717e50acba106b76c6c345192274ddc06.

g3 is updated to allow this to land

BUG=skia:

Change-Id: I5f4b06c2dc77dfa66d9b32348375151177511a65
Reviewed-on: https://skia-review.googlesource.com/5406
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
deleted file mode 100644
index 199dff2..0000000
--- a/include/core/SkXfermode.h
+++ /dev/null
@@ -1,13 +0,0 @@
-/*
- * Copyright 2006 The Android Open Source Project
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#ifndef SkXfermode_DEFINED
-#define SkXfermode_DEFINED
-
-//#error "SkXfermode.h is deprecated"
-
-#endif