Get rid of CiviCRM php5.6 composer tests

We're upgrading some dependencies so we won't support 5.6 any more.

We've still got one test to run PHPUnit (wikimedia-fundraising-civicrm-docker)
but this leaves us without a working test suite running our linters.

Change-Id: I552bd814ca00c19ffc1b2c99a375694d2327e5b3
TODO: replace them with php7.0 (https://phabricator.wikimedia.org/T230446)