Remove Rift and Necro from Mixed servers

Reason beeing that these maps tend to have players *purposely* stalling the game, *purposely* making the game **not end**, so they can farm kills with their OP gear they bought using the emeralds they infinitely get by killing players that have the spawnkit.

Especially on EU Alpha, I would find this a problem because:
* Not very large player count
* Only 1 (one) mixed gamemode server