Use new first-class MySQL timeout support in Phabricator

Summary: Fixes T6710. After D15669, we support a proper timeout parameter, so we don't need this hack anymore.

Test Plan: See D15669: forced a MySQL connector, set a low timeout, set a bad database, saw fast failures.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6710

Differential Revision: https://secure.phabricator.com/D15670