drm/amdgpu: drop unused function

amdgpu_discovery_get_ip_version() has not been used since
commit c40bdfb2 ("drm/amdgpu: fix incorrect VCN revision in SRIOV")
so drop it.

Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>