Ensure object-clause mapper works.

Per [bug 68204240](https://www.pivotaltracker.com/story/show/68204240); follow-up to [PR 1135](https://github.com/reciprocity/ggrc-core/pull/1135).  Found via ARTY.

Object deletion was failing because it was missing a way to create an object/clause mapper under the new clause/section setup.  This adds the necessary attributes to `sectionable` to cover that part.