drm/amdgpu/swsmu: clean up a bunch of stale interfaces

These were leftover from the initial implementation, but
never used.  Drop them.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Noticed-by: Ryan Taylor <ryan.taylor@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>