Fix ram leak

[CyriacF]

Fix ram leak with the PostLogin task never removed

[CyriacF]

[breitwan]

I think in this case it is better to do it in one operation.

@Subscribe
public void onPostLogin(PostLoginEvent event) {
  UUID uuid = event.getPlayer().getUniqueId();
  final Runnable postLoginTask = plugin.getPostLoginTasks().remove(uuid);
  if (postLoginTask != null) {
    this.plugin.getServer().getScheduler()
        .buildTask(this.plugin, () -> postLoginTask.run())
        .delay(Settings.IMP.MAIN.PREMIUM_AND_FLOODGATE_MESSAGES_DELAY, TimeUnit.MILLISECONDS)
        .schedule();
  }
}