ItemBlock: drop the charade about overriding built-in block types

this allows cleaning up a whole lot of abusable mess from the API, and we never properly supported overriding built-in block types anyway.

src/block/Sapling.php

@@ -46,7 +46,7 @@ class Sapling extends Flowable{
 		$this->treeType = $treeType;
 	}
 
-	protected function describeState(RuntimeDataDescriber $w) : void{
+	public function describeState(RuntimeDataDescriber $w) : void{
 		$w->bool($this->ready);
 	}