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/Sponge.php

@@ -28,7 +28,7 @@ use pocketmine\data\runtime\RuntimeDataDescriber;
 class Sponge extends Opaque{
 	protected bool $wet = false;
 
-	protected function describeType(RuntimeDataDescriber $w) : void{
+	public function describeType(RuntimeDataDescriber $w) : void{
 		$w->bool($this->wet);
 	}