Remove unwanted code for writing png

`info_ptr->channels` will be set in the next called `png_set_IHDR()`,
so it is no effect.

Signed-off-by: OBATA Akio <obache@wizdas.com>

src/kernel/qpngio.cpp

@@ -568,19 +568,6 @@ bool QPNGImageWriter::writeImage(const QImage& image, int quality_in, int off_x_
 
     png_set_write_fn(png_ptr, (void*)this, qpiw_write_fn, qpiw_flush_fn);
 
-#if PNG_LIBPNG_VER>=10500
-#warning XXXtnn not too sure about this
-/* 
-according to png.h, channels is only used on read, not writes, so we
-should be able to comment this out.
-*/
-#else /* LIBPNG 1.5 */
-    info_ptr->channels =
-	(image.depth() == 32)
-	    ? (image.hasAlphaBuffer() ? 4 : 3)
-	    : 1;
-#endif /* LIBPNG 1.5 */
-
     png_set_IHDR(png_ptr, info_ptr, image.width(), image.height(),
 	image.depth() == 1 ? 1 : 8 /* per channel */,
 	image.depth() == 32