revert removed function to avoid regression

Author: wxiaoguang

models/issues/stopwatch.go

@@ -46,6 +46,11 @@ func (s Stopwatch) Seconds() int64 {
 	return int64(timeutil.TimeStampNow() - s.CreatedUnix)
 }
 
+// Duration returns a human-readable duration string based on local server time
+func (s Stopwatch) Duration() string {
+	return util.SecToHours(s.Seconds())
+}
+
 func getStopwatch(ctx context.Context, userID, issueID int64) (sw *Stopwatch, exists bool, err error) {
 	sw = new(Stopwatch)
 	exists, err = db.GetEngine(ctx).