drm/msm: drop A6xx GMU header

Now as the headers are generated during the build step, drop
pre-generated copies of the Adreno A6xx GMU header.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/585870/
Link: https://lore.kernel.org/r/20240401-fd-xml-shipped-v5-16-4bdb277a85a1@linaro.org