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I want to delete all entities from world. And if a tag or a group is registered for the entity, I want to remove them too. Like there was no such entity at all. It is much like World.delete function, but you should loop on all entities. I can't find a way. And can't believe the designers didn't prepare such functionality for clearing the world from entities. Do I miss something?

There's no default way of deleting all entities from the World - typically this is done by disposing and recreating the world - but it can be easily achieved by adding a custom manager:
public final class EntityTracker extends Manager {
private Bag<Entity> entities = new Bag<Entity>();
#Override
public void added(Entity e) {
entities.add(e);
}
#Override
public void deleted(Entity e) {
entities.remove(e);
}
public void deleteAllEntities() {
for (Entity e : entities)
e.deleteFromWorld();
}
}
In recent versions of artemis-odb it's easier to use the AspectSubscriptionManager:
IntBag entities = world.getAspectSubscriptionManager()
.get(Aspect.all())
.getEntities();
int[] ids = entities.getData();
for (int i = 0, s = entities.size(); s > i; i++) {
world.delete(ids[i]);
}
edit: The above code assumes artemis-odb; I'm not sure whether vanilla artemis' TagManager and GroupManager automatically removes entities upon deletion.