Make invoice creation more flexible

Decision about when to create invoices should be made by developer. So here
removed all parts related to automatic creation of invoices.

* Removed `OSCAR_INVOICES_GENERATE_AFTER_ORDER_PLACED` setting.
* Removed `ready` method from app config.
* Deleted `receivers.py`.

oscar_invoices/app_settings.py

@@ -4,6 +4,4 @@
 
 OSCAR_INVOICES_UPLOAD_FOLDER = getattr(settings, 'OSCAR_INVOICES_UPLOAD_FOLDER', 'invoices/%Y/%m/')
 
-OSCAR_INVOICES_GENERATE_AFTER_ORDER_PLACED = getattr(settings, 'OSCAR_INVOICES_GENERATE_AFTER_ORDER_PLACED', False)
-
 OSCAR_INVOICE_INVOICE_MODEL = getattr(settings, 'OSCAR_INVOICE_INVOICE_MODEL', 'oscar_invoices.Invoice')

oscar_invoices/config.py

@@ -6,6 +6,3 @@ class InvoicesConfig(AppConfig):
     label = 'oscar_invoices'
     name = 'oscar_invoices'
     verbose_name = _('Invoices')
-
-    def ready(self):
-        from . import receivers  # noqa