Remove the Cassandra module

It uses import-from-derivation, which is a bad thing, because this
causes hydra-evaluator to build Cassandra at evaluation time.

  $ nix-instantiate nixos/release.nix -A tests.cassandra.i686-linux --dry-run
  error: cannot read ‘/nix/store/c41blyjz6pfvk9fnvrn6miihq5w3j0l4-cassandra-2.0.16/conf/cassandra-env.sh’, since path ‘/nix/store/0j9ax4z8xhaz5lhrwl3bwj10waxs3hgy-cassandra-2.0.16.drv’ is not valid, at /home/eelco/Dev/nixpkgs/nixos/modules/services/databases/cassandra.nix:373:11

Also, the module is a mess (bad option descriptions, poor indentation,
a gazillion options where a generic "config" option would suffice, it
opens ports in the firewall, it sets vm.swappiness, ...).