Implement DestructibleInterface on GitLFS refs

Summary: Ref T7789. Make sure these get cleaned up when a repository is destroyed.

Test Plan:

• Created a new repository.
• Pushed some LFS data to it.
• Used bin/remove destroy to nuke it.
• Verified the LFS stuff was cleaned up and the underlying files were destroyed (SELECT * FROM repository_gitlfsref, etc).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D15493