Remove SK_ENABLE_LIBPNG define as it is no longer needed by Android.

src/images/SkImageDecoder_Factory.cpp

Index: src/images/SkImageDecoder_Factory.cpp
diff --git a/src/images/SkImageDecoder_Factory.cpp b/src/images/SkImageDecoder_Factory.cpp
index f3cb120a477f822426a5ea63958085e88679ee8e..5640e0d9df90d5e53fbbaf01710733edfddf249b 100644
--- a/src/images/SkImageDecoder_Factory.cpp
+++ b/src/images/SkImageDecoder_Factory.cpp
@@ -18,10 +18,6 @@ typedef SkTRegistry<SkImageDecoder*, SkStream*> DecodeReg;
 // corner cases.
 template DecodeReg* SkTRegistry<SkImageDecoder*, SkStream*>::gHead;
 
-#ifdef SK_ENABLE_LIBPNG
-    extern SkImageDecoder* sk_libpng_dfactory(SkStream*);
-#endif
-
 SkImageDecoder* SkImageDecoder::Factory(SkStream* stream) {
     SkImageDecoder* codec = NULL;
     const DecodeReg* curr = DecodeReg::Head();
@@ -35,13 +31,6 @@ SkImageDecoder* SkImageDecoder::Factory(SkStream* stream) {
         }
         curr = curr->next();
     }
-#ifdef SK_ENABLE_LIBPNG
-    codec = sk_libpng_dfactory(stream);
-    stream->rewind();
-    if (codec) {
-        return codec;
-    }
-#endif
     return NULL;
 }