Do not use super().get_urlpatterns in ModelViewSet and SnippetViewSet

In ModelViewSet, this doesn't do anything as the base ViewSet class
returns an empty list.

In SnippetViewSet, this causes the ModelViewSet's urlpatterns to be
carried over, resulting in duplicate URLs that we do not want.

wagtail/admin/viewsets/model.py

@@ -409,7 +409,7 @@ class ModelViewSet(ViewSet):
             ReferenceIndex.register_model(self.model)
 
     def get_urlpatterns(self):
-        return super().get_urlpatterns() + [
+        return [
             path("", self.index_view, name="index"),
             path("results/", self.index_results_view, name="index_results"),
             path("new/", self.add_view, name="add"),

wagtail/snippets/views/snippets.py

@@ -1255,7 +1255,7 @@ class SnippetViewSet(ModelViewSet):
         )
 
     def get_urlpatterns(self):
-        urlpatterns = super().get_urlpatterns() + [
+        urlpatterns = [
             path("", self.index_view, name="list"),
             path("results/", self.index_results_view, name="list_results"),
             path("add/", self.add_view, name="add"),