Remove now fixed flatWorldStructureSpawning rule and cleanup some unused mixins

src/main/java/carpet/fakes/PlacedFeatureInterface.java

@@ -1,7 +0,0 @@
-package carpet.fakes;
-
-import net.minecraft.world.level.levelgen.feature.ConfiguredFeature;
-
-public interface PlacedFeatureInterface {
-    //ConfiguredFeature<?, ?> getRawFeature();
-}