drm/amdgpu: fixed the warnings reported by kernel test robot

The reported warnings are as follows:
  1.warning:no-previous-prototype-for-amdgpu_hdp_ras_fini.
  2.warning:no-previous-prototype-for-amdgpu_mmhub_ras_fini.

Amdgpu_hdp_ras_fini and amdgpu_mmhub_ras_fini are unused
in the code, they are the only functions in amdgpu_hdp.c
and amdgpu_mmhub.c. After removing these two functions,
both amdgpu_hdp.c and amdgpu_mmhub.c are empty, so these
two files can be deleted to fix the warning.

Signed-off-by: yipechai <YiPeng.Chai@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>