Remove maintainership

With this patch I remove myself as a maintainer for all packages I
currently maintain.

This is due the fact that I will be basically off the grid from May 2018
until early 2019, as I will be on a trip through north america.

I will revert this patch as soon as I'm back, as I plan to continue
contributing to nixpkgs then.
But as I cannot maintain anything during that time, I'd like to get this
patch merged.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>