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/Durable.php

@@ -82,6 +82,14 @@ abstract class Durable extends Item{
 		return 0;
 	}
 
+
+	/**
+	 * Returns the maximum amount of damage this item can take before it breaks.
+	 *
+	 * @return int
+	 */
+	abstract public function getMaxDurability() : int;
+
 	/**
 	 * Returns whether the item is broken.
 	 * @return bool