fix code dup line

PegasusWang · PegasusWang · commit 7b791b7a202d · 2018-05-12T18:19:53.000+08:00
diff --git a/docs/09_集合/set_adt.py b/docs/09_集合/set_adt.py
@@ -158,7 +158,6 @@ def __and__(self, other_set):
     def __sub__(self, other_set):
         """差集 A-B"""
         new_set = SetADT()
-        new_set = SetADT()
         for element_a in self:
             if element_a not in other_set:
                 new_set.add(element_a)
