Default and flowers: only run on-generated functions in mgv6. Remove indev code

I just discovered that default and flowers mods are running slow and unnecessary on-generation functions in mgv5, mgv7 and singlenode mapgens. Mgv5, mgv6 and future mapgens will be using the one biome API with all flora placed using the decoration code.
Nyancat generation is moved to a separate on-generated function to continue in all mapgens, perhaps those should be mgv6 only what do you think?, i don't mind either way.
I also found some indev code to delete.
You can pass `default.mgv6ongen` directly to `register_on_generated` instead of proxying it.

Also, I'd name it `mgv6_ongen`...