Use the built-in json module

Python >= 2.6 comes with json by default. simplejson is only useful for Python <= 2.5.

There are still occurrences in `thirdpart/{captcha,rest_framework}` but they use simplejson from `django.utils` which is available in Django 1.5.

Corresponding seafile PR: https://github.com/haiwen/seafile/pull/723