drm/msm: drop C++ parts of a6xx.xml.h

As a preparation to removal of a6xx.xml.h, drop the C++ part of the
heder, it is not used by the kernel anyway.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/585866/
Link: https://lore.kernel.org/r/20240401-fd-xml-shipped-v5-17-4bdb277a85a1@linaro.org