travis: drop archlinux from Travis cron builds

travis: drop archlinux from Travis cron builds

With Archlinux being a rolling release every new build of the docker
image could contain new changes that would break for us. For all other
distros we also follow the latest release approach where we want to make
sure efl still builds for it. With Archlinux this is not possible by its
nature. Luckily enough efl developers use Archlinux so the risk of
issues being left unnoticed is small enough.

Differential Revision: https://phab.enlightenment.org/D7306