mei: trace: remove unused TRACE_SYSTEM_STRING

fix warning:
include/trace/ftrace.h:28:0: note: this is the location of the previous definition
 ^
In file included from include/trace/define_trace.h:90:0,
                 from drivers/misc/mei/mei-trace.h:76,
                 from drivers/misc/mei/mei-trace.c:21:
include/trace/ftrace.h:28:0: warning: "TRACE_SYSTEM_STRING" redefined

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/mei-trace.h

@@ -24,9 +24,7 @@
 #include <linux/device.h>
 
 #undef TRACE_SYSTEM
-
 #define TRACE_SYSTEM mei
-#define TRACE_SYSTEM_STRING __stringify(TRACE_SYSTEM)
 
 TRACE_EVENT(mei_reg_read,
 	TP_PROTO(const struct device *dev, const char *reg, u32 offs, u32 val),