remove double including of libpng decoder

gyp/images.gyp

 # GYP file for images project.
 {
   'targets': [
     {
       'target_name': 'images',
       'product_name': 'skia_images',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'libjpeg.gyp:*',
@@ skipping 88 matching lines @@
         },{ #else if skia_os != mac
           'sources!': [
             '../src/ports/SkImageDecoder_CG.cpp',
           ],
         }],
         [ 'skia_os in ["linux", "freebsd", "openbsd", "solaris"]', {
           # Any targets that depend on this target should link in libpng, libgif, and
           # our code that calls it.
           # See http://code.google.com/p/gyp/wiki/InputFormatReference#Dependent_Settings
           'link_settings': {
-            'sources': [
-              '../src/images/SkImageDecoder_libpng.cpp',
-            ],
             'libraries': [
               '-lgif',
               '-lpng',
               '-lz',
             ],
           },
           # end libpng/libgif stuff
         }],
         # FIXME: NaCl should be just like linux, etc, above, but it currently is separated out
         # to remove gif. Once gif is supported by naclports, this can be merged into the above
         # condition.
         [ 'skia_os == "nacl"', {
           'sources!': [
             '../src/images/SkImageDecoder_libgif.cpp',
             '../src/images/SkMovie_gif.cpp',
           ],
           'link_settings': {
-            'sources': [
-              '../src/images/SkImageDecoder_libpng.cpp',
-            ],
             'libraries': [
               '-lpng',
               '-lz',
             ],
           },
         }],
         [ 'skia_os == "android"', {
           'include_dirs': [
              '../src/utils',
           ],
@@ skipping 31 matching lines @@
       },
     },
   ],
 }
 
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