Remove unused code

openfisca_core/periods.py

@@ -9,11 +9,7 @@
 Since a period is a triple it can be used as a dictionary key.
 """
 
-from __future__ import unicode_literals, print_function, division, absolute_import
-from builtins import str
-
 import calendar
-import collections
 import datetime
 import re
 from os import linesep
@@ -706,15 +702,6 @@ def stop(self):
                     day -= month_last_day
         return Instant((year, month, day))
 
-    def to_json_dict(self):
-        if not isinstance(self[1], str):
-            self[1] = to_unicode(self[1])
-        return collections.OrderedDict((
-            ('unit', self[0]),
-            ('start', self[1]),
-            ('size', self[2]),
-            ))
-
     @property
     def unit(self):
         return self[0]