Remove the legacy chart behavior from Maniphest

Summary: Depends on D20486. Ref T13279. Now that the "Reports" UI uses a panel to draw a real chart from Facts, throw away the copy of the old code.

Test Plan: grep

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20487