libgd: remove unused LIBPNG12_CONFIG definition

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>

libs/libgd/Makefile

@@ -54,7 +54,6 @@ CONFIGURE_ARGS += \
 	--without-iconv
 
 CONFIGURE_VARS += \
-	LIBPNG12_CONFIG="$(STAGING_DIR_HOSTPKG)/bin/libpng12-config" \
 	ac_cv_header_iconv_h=no
 
 define Build/InstallDev