Fixed deprecation warning for OfflinePlayer.setBanned

There is no such method called BanList.unban(String).
The Method is called BanList.pardon(String)
See https://github.com/Bukkit/Bukkit/commit/75427e084d0f2a444375324b0ef7334aa66d93be#diff-972301d60a220127554416f7171487bfR50