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>

drivers/gpu/drm/amd/amdgpu/amdgpu_trace.h

@@ -9,7 +9,6 @@
 
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM amdgpu
-#define TRACE_SYSTEM_STRING __stringify(TRACE_SYSTEM)
 #define TRACE_INCLUDE_FILE amdgpu_trace
 
 TRACE_EVENT(amdgpu_bo_create,