Remove obsolete columns from RefCursor table

Summary:
Ref T11823. This change isn't standalone, but prepares for the more involved code change by dropping obsolete columns from the RefCursor table and adding the unique key we need to prevent the ambiguous/duplicate refs issue.

This data was moved to the RefPosition table in D18612.

Test Plan: Ran storage upgrade. See next revision for more substantial testing of this change series.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11823

Differential Revision: https://secure.phabricator.com/D18613