[B] JavaPlugin.getDefaultWorldGenerator should not print a message

Only plugins are going to be calling this method. Thus, only plugins should be notified of the result (null). No need to tell the user that too.
See MultiVerse for an example of where this is used in practise.
https://bukkit.atlassian.net/browse/BUKKIT-2565
Bukkit/CraftBukkit#1082