Fix #408 Undo works only once

(it worked only 1 time, then go in cycles on the latest backup)

Close https://github.com/wkentaro/labelme/pull/409

labelme/widgets/canvas.py

@@ -109,7 +109,6 @@ class Canvas(QtWidgets.QWidget):
         self.shapesBackups.pop()  # latest
         shapesBackup = self.shapesBackups.pop()
         self.shapes = shapesBackup
-        self.storeShapes()
         self.repaint()
 
     def enterEvent(self, ev):