drm/amdgpu: Remove unused nbif_v6_3_1_sriov_funcs

The nbif_v6_3_1_sriov_funcs instance of amdgpu_nbio_funcs was added in
commit 894c6d35 ("drm/amdgpu: Add nbif v6_3_1 ip block support")
but has remained unused.

Alex has confirmed it wasn't needed.

Remove it, together with the four unused stub functions:
  nbif_v6_3_1_sriov_ih_doorbell_range
  nbif_v6_3_1_sriov_gc_doorbell_init
  nbif_v6_3_1_sriov_vcn_doorbell_range
  nbif_v6_3_1_sriov_sdma_doorbell_range

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>