drm/amdgpu: remove unused TRACE_SYSTEM_STRING define

Port of 77cb2fea1e5fc4b083dd967f231bbf6edd96150e to
amdgpu.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
1 file changed