Remove outdated comments about setting up permissions to only give ce…

…rtain buttons

These were only relevant when the assertions were retrieving the relevant buttons by index.

These were only meaningful when

wagtail/admin/tests/test_buttons_hooks.py

@@ -251,7 +251,6 @@ def test_delete_button_with_next_url(self):
         Ensure that the built in delete button supports a next_url provided.
         """
 
-        # page_listing_more_button generator yields only `Delete button` with this permission set
         page_perms = DeleteOnlyPagePerms()
         page = self.root_page
         base_url = reverse("wagtailadmin_pages:delete", args=[page.id])
@@ -271,7 +270,6 @@ def test_delete_button_with_invalid_next_url(self):
         As the page is now deleted and cannot be edited.
         """
 
-        # permissions should yield two buttons, delete and unpublish
         page_perms = DeleteAndUnpublishPagePerms()
         page = self.root_page