Revert "Implement ExtendedPropertyValueSnak"

Author: JeroenDeDauw

src/Snak/DerivedPropertyValueSnak.php

@@ -18,7 +18,7 @@
  */
 class PropertyValueSnak extends SnakObject {
 
-	protected $dataValue;
+	private $dataValue;
 
 	/**
 	 * Support for passing in an EntityId instance that is not a PropertyId instance has