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

src/images/SkImageEncoder_Factory.cpp

Index: src/images/SkImageEncoder_Factory.cpp
diff --git a/src/images/SkImageEncoder_Factory.cpp b/src/images/SkImageEncoder_Factory.cpp
index 2bd1113f7c9512dee48b2d53553e14d4d22baffa..10f8d64decffaa6bad6af71bd6d6fa4ba697e162 100644
--- a/src/images/SkImageEncoder_Factory.cpp
+++ b/src/images/SkImageEncoder_Factory.cpp
@@ -15,10 +15,6 @@ typedef SkTRegistry<SkImageEncoder*, SkImageEncoder::Type> EncodeReg;
 // Can't use the typedef here because of complex C++ corner cases
 template EncodeReg* SkTRegistry<SkImageEncoder*, SkImageEncoder::Type>::gHead;
 
-#ifdef SK_ENABLE_LIBPNG
-    extern SkImageEncoder* sk_libpng_efactory(SkImageEncoder::Type);
-#endif
-
 SkImageEncoder* SkImageEncoder::Create(Type t) {
     SkImageEncoder* codec = NULL;
     const EncodeReg* curr = EncodeReg::Head();
@@ -28,10 +24,5 @@ SkImageEncoder* SkImageEncoder::Create(Type t) {
         }
         curr = curr->next();
     }
-#ifdef SK_ENABLE_LIBPNG
-    if ((codec = sk_libpng_efactory(t)) != NULL) {
-        return codec;
-    }
-#endif
     return NULL;
 }