Change `app_settings.PERIODIC_TASK_SUFFIX` default to an empty string

[joshuadavidthomas]

Since we've moved to tracking the registered task via a Task model, there's not really a need to have the suffix on the Schedule -- it was used to filter out schedules that were not managed by django-q-registry.

I still think it might be useful to still expose the setting for users to opt-in; some people may want to see at a glance in the admin of schedules managed by this package. But it should default to be empty.