Remove unused interface

Note: release notes do not need updating due to interface being
in the internal namespace already

Author: JeroenDeDauw

src/Internal/MapHasher.php

@@ -14,7 +14,7 @@
  * @licence GNU GPL v2+
  * @author Jeroen De Dauw < jeroendedauw@gmail.com >
  */
-class MapValueHasher implements MapHasher {
+class MapValueHasher {
 
 	private $isOrdered;