Remove legacy "touched" table and indexing

Summary: Noticed this in the schema. "Touches" were an idea that never really got off the ground, as we built out more/better notification channels instead. Essentially, they recorded any object you'd ever interacted with. Maybe this will be useful some day, but for now it does nothing and can't be interacted with. Nuke it.

Test Plan: `grep`, loaded Maniphest.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6953