devel/godot2: stop using builtin libpng

platform/x11/detect.py actually forces builtin libpng if builtin freetype or zlib is used.

Use the system ones.

This also fixes build on powerpc64le, since builtin libpng is not built with
VSX.

PR: 254703
Approved by: FreeBSD@ShaneWare.Biz (maintainer)