drm/amd/pp: Delete dead code in powerplay

As not support per DPM level optimization,
so delete activity_target array.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>