Fix PhutilOAuth1FutureTestCase::testOAuth1SigningWithJIRAExamples for PHP 8

Summary:
PHP 8 deprecates openssl_free_key as the key is automatically freed, so
silence the warning in PhutilOAuth1Future::signString.

Test Plan: Ran arc lint --everything

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21502