Remove XML_GLOBAL_NAMESPACE (php#14265)

This constant is only available if it is defined by libxml2, but it is
never defined because the minimum version of libxml2 that we support had
removed XML_GLOBAL_NAMESPACE already.

Author: ndossche

ext/dom/php_dom.stub.php

@@ -94,14 +94,6 @@
      * @cvalue XML_NAMESPACE_DECL
      */
     const XML_NAMESPACE_DECL_NODE = UNKNOWN;
-#ifdef XML_GLOBAL_NAMESPACE
-    /**
-     * @var int
-     * @cvalue XML_GLOBAL_NAMESPACE
-     */
-    const XML_GLOBAL_NAMESPACE = UNKNOWN;
-#endif
-
     /**
      * @var int
      * @cvalue XML_LOCAL_NAMESPACE