Item: Moved getMaxDurability() to Durable class only, and make it abstract

It's only now used in the Durable class, so it does not make sense to keep it in Item anymore. This is a leftover from the days where Durable did not exist.

src/pocketmine/item/Bow.php

@@ -39,7 +39,7 @@ class Bow extends Tool{
 		return 200;
 	}
 
-	public function getMaxDurability(){
+	public function getMaxDurability() : int{
 		return 385;
 	}