Fixes: unwanted gcc export of symbols from bundled libpng

This removes a change to libpng that was only relevant for building
a certain Qt 4 tool in certain environements without system libpng.
This is no longer relevant, so this kludge should now be removed.

Change-Id: I9a04ab0224f9e84ccab3ec27762582be439fe3ef
Reviewed-by: Kim M. Kalland <kim.kalland@nokia.com>

src/3rdparty/libpng/pngconf.h

@@ -277,13 +277,6 @@
 #    endif
 #  endif
 
-#   if !defined(PNG_IMPEXP)
-#       include <qconfig.h>
-#       if defined(QT_VISIBILITY_AVAILABLE)
-#           define PNG_IMPEXP __attribute__((visibility("default")))
-#       endif
-#   endif
-
 #  ifndef PNG_IMPEXP
 #    define PNG_IMPEXP
 #  endif