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 <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/585870/
Link: https://lore.kernel.org/r/[email protected]

Author: lumag