Remove obsolete supportsAsyncQueries() check for MYSQLI_ASYNC
Summary:
This function only checks for `defined('MYSQLI_ASYNC')`. That is always true as the old, non-mysqlnd driver was removed in PHP 7.0.0 according to https://www.php.net/manual/en/intro.mysql.php, and rP23a49eb4 and rP7d8c84a7 bumped the required PHP version from 5.2.3 to 7.2.25.
This was a public function; probably worth to mention in the ChangeLog for third parties.
Closes T16235
Test Plan: Unknown.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16235
Differential Revision: https://we.phorge.it/D26289