Allow Legalpad documents to be destroyed with bin/remove destroy

Summary: Ref T3116. Support permanent destruction of legal document objects.

Test Plan: Ran bin/remove destroy L1, saw it clean up the document body, signatures, transactions and edges.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9775