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>