Less_Tree_Call: Remove unused value property

This has been here since the very first commit 810eb9f, but
was never used. It also does not exist in Less.js 2.5.3 or 3.13.1.

Change-Id: I18a4188745ce53ffae0d596578d94719d768ac03

lib/Less/Tree/Call.php

@@ -3,9 +3,7 @@
  * @private
  * @see less-3.13.1.js#Call.prototype
  */
-class Less_Tree_Call extends Less_Tree implements Less_Tree_HasValueProperty {
-	public $value;
-
+class Less_Tree_Call extends Less_Tree {
 	public $name;
 	public $args;
 	public $calc;