Don't warn about "always_populate_raw_post_data" on PHP7

Summary: Ref T9640. This option was removed in PHP7, so there's no reason to warn about it.

Test Plan: No longer saw a setup warning on PHP7.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640

Differential Revision: https://secure.phabricator.com/D17196