drm/amdgpu: remove unused function

amdgpu_boot_test_post_card() is not used anywhere.  Probably
a leftover from the original port from radeon.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>