R1:32225d1dd0f8

Remove three ancient columns from Maniphest tasks: attached, projectPHIDs, ccPHIDs

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…