Remove unused code in the admin

- remove `ActiveSchedulesFilter`,
- remove `ActiveShowsFilter`,
- replace `ActivityFilter` with `ActiveHostsFilter`, since it remains in use

program/admin.py

@@ -25,8 +25,9 @@ from django.utils.translation import gettext_lazy as _
 from .models import Language, Type, MusicFocus, Category, Topic, FundingCategory, Host
 
 
-class ActivityFilter(admin.SimpleListFilter):
+class ActiveHostsFilter(admin.SimpleListFilter):
     title = _("Activity")
+    parameter_name = 'has_shows_and_schedules'
 
     def lookups(self, request, model_admin):
         return (
@@ -35,35 +36,10 @@ class ActivityFilter(admin.SimpleListFilter):
         )
 
     def queryset(self, request, queryset):
-        if self.parameter_name == 'has_timeslots':  # active/inactive Schedules
-            if self.value() == 'yes':
-                return queryset.filter(until__gt=timezone.now()).distinct()
-            if self.value() == 'no':
-                return queryset.filter(until__lt=timezone.now()).distinct()
-
-        if self.parameter_name == 'has_schedules_timeslots':  # active/inactive Shows
-            if self.value() == 'yes':
-                return queryset.filter(schedules__until__gt=timezone.now()).distinct()
-            if self.value() == 'no':
-                return queryset.filter(schedules__until__lt=timezone.now()).distinct()
-
-        if self.parameter_name == 'has_shows_schedules_timeslots':  # active/inactive Hosts
-            if self.value() == 'yes':
-                return queryset.filter(shows__schedules__until__gt=timezone.now()).distinct()
-            if self.value() == 'no':
-                return queryset.filter(shows__schedules__until__lt=timezone.now()).distinct()
-
-
-class ActiveSchedulesFilter(ActivityFilter):
-    parameter_name = 'has_timeslots'
-
-
-class ActiveShowsFilter(ActivityFilter):
-    parameter_name = 'has_schedules_timeslots'
-
-
-class ActiveHostsFilter(ActivityFilter):
-    parameter_name = 'has_shows_schedules_timeslots'
+        if self.value() == 'yes':
+            return queryset.filter(shows__schedules__until__gt=timezone.now()).distinct()
+        if self.value() == 'no':
+            return queryset.filter(shows__schedules__until__lt=timezone.now()).distinct()
 
 
 class TypeAdmin(admin.ModelAdmin):