Remove Frando from MAINTAINERS.txt

While I am still following core development quite closely, though passively, I don't have the time to fullfill an active maintainer's role in these days. Especially since I started studying again (in non-technical fields) and also due to other non-Drupal committments my Drupal time apart from some client work is severly limited. Therefore, it is overdue to have myself removed from a sub-system maintainer's position for Drupal core. I surely won't be leaving the project, and only time will tell if Drupal will become a major life focus again sooner or later! Oh, and of course I enjoyed working on Drupal core massively and am very thankful for the amazing community, to which I owe a lot.