tracing/drm: Remove unused TRACE_SYSTEM_STRING define

The tracing infrastructure is adding a macro TRACE_SYSTEM_STRING, and
hit the following build failure:

   In file included from include/trace/define_trace.h:90:0,
                    from drivers/gpu/drm/.//radeon/radeon_trace.h:209,
                    from drivers/gpu/drm/.//radeon/radeon_trace_points.c:9:
>> include/trace/ftrace.h:28:0: warning: "TRACE_SYSTEM_STRING" redefined
    #define TRACE_SYSTEM_STRING __app(TRACE_SYSTEM_VAR,__trace_system_name)

Seems that the DRM folks have added their own use to the
TRACE_SYSTEM_STRING, with:

 #define TRACE_SYSTEM_STRING __stringify(TRACE_SYSTEM)

Although, I can not find its use anywhere. I could simply use another
name, but if this macro is not being used, it should be removed.

Link: http://lkml.kernel.org/r/20150402123736.01eda052@gandalf.local.home

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

drivers/gpu/drm/drm_trace.h

@@ -7,7 +7,6 @@
 
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM drm
-#define TRACE_SYSTEM_STRING __stringify(TRACE_SYSTEM)
 #define TRACE_INCLUDE_FILE drm_trace
 
 TRACE_EVENT(drm_vblank_event,

drivers/gpu/drm/i915/i915_trace.h

@@ -12,7 +12,6 @@
 
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM i915
-#define TRACE_SYSTEM_STRING __stringify(TRACE_SYSTEM)
 #define TRACE_INCLUDE_FILE i915_trace
 
 /* pipe updates */

drivers/gpu/drm/radeon/radeon_trace.h

@@ -9,7 +9,6 @@
 
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM radeon
-#define TRACE_SYSTEM_STRING __stringify(TRACE_SYSTEM)
 #define TRACE_INCLUDE_FILE radeon_trace
 
 TRACE_EVENT(radeon_bo_create,