signals: Drop provider args when creating Signal object

- Convert the information into code comments.

- This argument seems to be unused. Passing the argument has been deprecated. On
Django 3.2 it throws a warning that it will be removed from Django 4.0.

Tests:

- Run unit tests on Django 3.2 and Django 2.2. With Django 3.2, warning related
to provider_args are no longer show.

- On Django 3.2 and Django 2.2, change the hostname to trigger a signal.

Tests:

- Install ebabberd. Goto system -> config and change the hostname and domain
name. This triggers pre_hostname_change, post_hostname_change, domain_removed
and domain_added signals. There should be no errors during the process. Console
shows no errors.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/signals.py

@@ -5,11 +5,23 @@ Django signals emitted within FreedomBox.
 
 from django.dispatch import Signal
 
+# Arguments: -
 pre_module_loading = Signal()
+
+# Arguments: -
 post_module_loading = Signal()
-post_setup = Signal(providing_args=['module_name'])
-pre_hostname_change = Signal(providing_args=['old_hostname', 'new_hostname'])
-post_hostname_change = Signal(providing_args=['old_hostname', 'new_hostname'])
-domain_added = Signal(
-    providing_args=['domain_type', 'name', 'description', 'services'])
-domain_removed = Signal(providing_args=['domain_type', 'name'])
+
+# Arguments: module_name
+post_setup = Signal()
+
+# Arguments: old_hostname, new_hostname
+pre_hostname_change = Signal()
+
+# Arguments: old_hostname, new_hostname
+post_hostname_change = Signal()
+
+# Arguments: domain_type, name, description, services
+domain_added = Signal()
+
+# Arguments: domain_type, name
+domain_removed = Signal()