png: Allow building shared freetype with bundled libpng

This was the behaviour when building Godot 2.1, which allows to build against
Ubuntu 12.04 and its freetype that links old libpng12, while still bundling
libpng16.

platform/server/detect.py

@@ -76,7 +76,6 @@ def configure(env):
         env.ParseConfig('pkg-config libwebp --cflags --libs')
 
     if (env['builtin_freetype'] == 'no'):
-        env['builtin_libpng'] = 'no'  # Freetype links against libpng
         env.ParseConfig('pkg-config freetype2 --cflags --libs')
 
     if (env['builtin_libpng'] == 'no'):

platform/x11/detect.py

@@ -140,7 +140,6 @@ def configure(env):
         env.ParseConfig('pkg-config libwebp --cflags --libs')
 
     if (env['builtin_freetype'] == 'no'):
-        env['builtin_libpng'] = 'no'  # Freetype links against libpng
         env.ParseConfig('pkg-config freetype2 --cflags --libs')
 
     if (env['builtin_libpng'] == 'no'):