While fixing up some incorrect STAGING_DIR_HOSTPKG uses I introduced, I
noticed that the LIBPNG12_CONFIG variable is not used by libgd anymore (it
is called LIBPNG_CONFIG now).
As the build runs fine without setting LIBPNG_CONFIG, let's just remove the
unused line.
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>