removed 1st edition code

17-it-generator/tree/4steps/tree_step3.py

@@ -1,10 +0,0 @@
-def tree(cls, level=0):
-    yield cls.__name__, level
-    for sub_cls in cls.__subclasses__():
-        yield from tree(sub_cls, level + 1)
-
-
-if __name__ == '__main__':
-    for cls_name, level in tree(BaseException):
-        indent = ' ' * 4 * level
-        print(f'{indent}{cls_name}')