perf trace: Remove thread_trace->exit_time

Not used at all, we need just the entry_time to calculate the syscall
duration.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-js6r09zdwlzecvaei7t4l3vd@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-trace.c

@@ -843,7 +843,6 @@ static size_t fprintf_duration(unsigned long t, FILE *fp)
  */
 struct thread_trace {
 	u64		  entry_time;
-	u64		  exit_time;
 	bool		  entry_pending;
 	unsigned long	  nr_events;
 	unsigned long	  pfmaj, pfmin;
@@ -1571,8 +1570,6 @@ static int trace__sys_exit(struct trace *trace, struct perf_evsel *evsel,
 		++trace->stats.vfs_getname;
 	}
 
-	ttrace->exit_time = sample->time;
-
 	if (ttrace->entry_time) {
 		duration = sample->time - ttrace->entry_time;
 		if (trace__filter_duration(trace, duration))