avoid direct mutation of Item->count field, use Item->pop() instead

I think this change was already applied on the master branch, but I don't remember for sure.

src/pocketmine/block/Sugarcane.php

@@ -62,7 +62,7 @@ class Sugarcane extends Flowable{
 				$this->getLevel()->setBlock($this, $this, true);
 			}
 
-			$item->count--;
+			$item->pop();
 
 			return true;
 		}