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 <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/585866/
Link: https://lore.kernel.org/r/[email protected]

Author: lumag