Ensure that we document all signals

src/pretix/control/signals.py

@@ -2,16 +2,6 @@ from django.dispatch import Signal
 
 from pretix.base.signals import DeprecatedSignal, EventPluginSignal
 
-restriction_formset = EventPluginSignal(
-    providing_args=["item"]
-)
-"""
-This signal is sent out to build configuration forms for all restriction formsets
-(see plugin API documentation for details).
-
-As with all plugin signals, the ``sender`` keyword argument will contain the event.
-"""
-
 html_page_start = Signal(
     providing_args=[]
 )