Remove workaround in GLTF exporter that double converts ra textures to rg

(cherry picked from commit 4f41b9494353adb529af6c062c86361988481e27)

modules/gltf/gltf_document.cpp

@@ -3810,7 +3810,6 @@ Error GLTFDocument::_serialize_materials(Ref<GLTFState> p_state) {
 						}
 						img->decompress();
 						img->convert(Image::FORMAT_RGBA8);
-						img->convert_ra_rgba8_to_rg();
 						for (int32_t y = 0; y < img->get_height(); y++) {
 							for (int32_t x = 0; x < img->get_width(); x++) {
 								Color c = img->get_pixel(x, y);