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, these are making unwanted flora appear in mgv5 and mgv7 when default nodes are added using the biome API.
Mgv5, mgv7 and future mapgens will be using the biome API.
Nyancat generation is moved to a separate on-generated function to continue in all mapgens.
I also found some indev mapgen code to delete.