remove unused SkAvoidXfermode

gyp/effects.gypi

 # Include this gypi to include all 'effects' files
 # The parent gyp/gypi file must define:
 #       'skia_src_path'     e.g. skia/trunk/src
 #       'skia_include_path' e.g. skia/trunk/include
 #
 # The skia build defines these in common_variables.gypi.
 #
 {
   'sources': [
     '<(skia_src_path)/effects/Sk1DPathEffect.cpp',
     '<(skia_src_path)/effects/Sk2DPathEffect.cpp',
     '<(skia_src_path)/effects/SkAlphaThresholdFilter.cpp',
     '<(skia_src_path)/effects/SkArcToPathEffect.cpp',
     '<(skia_src_path)/effects/SkArithmeticMode.cpp',
     '<(skia_src_path)/effects/SkArithmeticMode_gpu.cpp',
     '<(skia_src_path)/effects/SkArithmeticMode_gpu.h',
-    '<(skia_src_path)/effects/SkAvoidXfermode.cpp',
     '<(skia_src_path)/effects/SkBitmapSource.cpp',
     '<(skia_src_path)/effects/SkBlurDrawLooper.cpp',
     '<(skia_src_path)/effects/SkBlurMask.cpp',
     '<(skia_src_path)/effects/SkBlurMask.h',
     '<(skia_src_path)/effects/SkBlurImageFilter.cpp',
     '<(skia_src_path)/effects/SkBlurMaskFilter.cpp',
     '<(skia_src_path)/effects/SkColorCubeFilter.cpp',
     '<(skia_src_path)/effects/SkColorFilters.cpp',
     '<(skia_src_path)/effects/SkColorFilterImageFilter.cpp',
     '<(skia_src_path)/effects/SkColorMatrix.cpp',
@@ skipping 88 matching lines @@
     '<(skia_include_path)/effects/SkPixelXorXfermode.h',
     '<(skia_include_path)/effects/SkPorterDuff.h',
     '<(skia_include_path)/effects/SkRectShaderImageFilter.h',
     '<(skia_include_path)/effects/SkTableColorFilter.h',
     '<(skia_include_path)/effects/SkTableMaskFilter.h',
     '<(skia_include_path)/effects/SkTileImageFilter.h',
     '<(skia_include_path)/effects/SkTransparentShader.h',
     '<(skia_include_path)/effects/SkMagnifierImageFilter.h',
   ],
 }