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: chad

Differential Revision: https://secure.phabricator.com/D15216