Remove flag needed by outdated libpng

BUG=skia:

Review URL: https://codereview.chromium.org/1514873003
diff --git a/gyp/images.gyp b/gyp/images.gyp
index 6f11514..1c434ae 100644
--- a/gyp/images.gyp
+++ b/gyp/images.gyp
@@ -125,13 +125,6 @@
           'include_dirs': [
              '../src/utils',
           ],
-          'cflags' : [
-            # SkImageDecoder_libpng includes png.h.
-            # In the version of libpng that we use on Android (1.2.46),
-            # there is a missing space between a literal and an identifier
-            # in png.h, triggering a warning in C++11.
-            '-Wno-literal-suffix',
-          ],
           'dependencies': [
             'libpng.gyp:libpng',
           ],