Fix incorrect event handler registration

[Tictim]

What

This PR fixes a part of event handler registration broken in a recent commit. Additionally, this PR removes one unnecessary registration.

Outcome

worldgen.addLoot option and worldgen.increaseDungeonLoot option, which broke due to the incorrect event handler registration, will work again.

Additional Information

I've looked into every occurrence of MinecraftForge.EVENT_BUS.register on the project and deemed there was no further issues. At least on surface level.

[ALongStringOfNumbers]

The chest gen hooks issue was probably because Tech made the subscribeEvent static in his probable bugs PR. Does simply making the SubscribeEvent not static fix the issue?

[Tictim]

I chose to modify the registration method and keep the event handler static because there was no usage of ChestGenHooks instance anywhere except on event registration. In fact the event handler being nonstatic were probably how IDEA complained about it in the first place. Keeping the event handler static should be enough to prevent same mistake happening in future.