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