Remove HHVM PHP-Mode

The HHVM developers announced to drop PHP support in HHVM. ISPConfig currently offers HHVM as PHP runtime option, this does not make much sense anymore when HHVM stops supporting PHP code, so we should remove the HHVM PHP mode option in ISPConfig.

HHVM as PHP mode option was introduced in ISPConfig at a time where HHVM was able to process PHP mode much faster than php 5.x. With PHP 7.x, PHP speed from Zend PHP engine got much faster, so there is no real need for an HHVM PHP moe anymore.

HHVM Team announcement.

https://hhvm.com/blog/2018/09/12/end-of-php-support-future-of-hack.html