Remove the Futures() method

Summary: Fixes T6829. Removes the Futures() method in favor of calling new FutureIterator($futures) explicitly. Depends on D11077, D11078 and D11079.

Test Plan: We could possibly deprecate this function instead (with phutil_deprecated).

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6829

Differential Revision: https://secure.phabricator.com/D11081