Remove django-celery-beat and use the default scheduler

There are some issues with timezone for the `DatabaseScheduler`, so we
fallback to the default one (celery.beat.PersistentScheduler) which is
simpler --doesn't allow us to change the scheduled task from the
admin, but we don't need that.

In case we need this again, we can come back when those issues are
already fixed and revert this commit.

readthedocs/settings/base.py

@@ -84,7 +84,6 @@ def INSTALLED_APPS(self):  # noqa
             'annoying',
             'django_extensions',
             'messages_extends',
-            'django_celery_beat',
 
             # daniellindsleyrocksdahouse
             'haystack',
@@ -245,7 +244,6 @@ def USE_PROMOS(self):  # noqa
     CELERY_CREATE_MISSING_QUEUES = True
 
     CELERY_DEFAULT_QUEUE = 'celery'
-    CELERYBEAT_SCHEDULER = 'django_celery_beat.schedulers:DatabaseScheduler'
     CELERYBEAT_SCHEDULE = {
         # Ran every hour on minute 30
         'hourly-remove-orphan-symlinks': {