Summary:
Before edges, we stored some of this stuff directly on tasks.
- `attached` was migrated to edges in Jan 2013.
- `projectPHIDs` was never used, as far as I can tell?
- `ccPHIDs` was migrated away and dropped more than a year ago.
None of these columns are used in modern code (instead, modern code uses edges).
Test Plan: `grep`, browsed around, `bin/storage upgrade`, unit tests.
Reviewers: chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D15216
