cleanup block dual bounding box clusterfuck

"bounding box" serves no tangible purpose, only collision boxes do right now.

src/block/Flowable.php

@@ -35,7 +35,10 @@ abstract class Flowable extends Transparent{
 		return false;
 	}
 
-	protected function recalculateBoundingBox() : ?AxisAlignedBB{
-		return null;
+	/**
+	 * @return AxisAlignedBB[]
+	 */
+	protected function recalculateCollisionBoxes() : array{
+		return [];
 	}
 }