php: drop 7.1

PHP 7.1 is currently on life support, as in only recieving security related patches.

This will only continue until: 2019-12-01

This date are in the middle of the 19.09 lifecycle. So it would be
nice to not have it in the 19.09 stable release. Dropping it now would
also result in less maintanance in updating them.

The death dates can be seen on following links:
 - https://endoflife.date/php
 - https://php.net/supported-versions.php
 - https://en.wikipedia.org/wiki/PHP#Release_history