delete redundant code (#2318)

I read `x2coco.py`, and I think `getbbox` function is redundant.

tools/x2coco.py

@@ -45,12 +45,6 @@ class MyEncoder(json.JSONEncoder):
             return super(MyEncoder, self).default(obj)
         
         
-def getbbox(self, points):
-    polygons = points
-    mask = self.polygons_to_mask([self.height, self.width], polygons)
-    return self.mask2box(mask)
-
-
 def images_labelme(data, num):
     image = {}
     image['height'] = data['imageHeight']