Fix contents parameter reassignment (#235)

Remove `self.contents = contents` from `FlexSendMessage.__init__` method because it didn't reference to another parameters before assign it again

Author: sheiun

linebot/models/flex_message.py

@@ -46,7 +46,6 @@ def __init__(self, alt_text=None, contents=None, **kwargs):
 
         self.type = 'flex'
         self.alt_text = alt_text
-        self.contents = contents
         self.contents = self.get_or_new_from_json_dict_with_types(
             contents, {
                 'bubble': BubbleContainer,