Remove monitor key field (#3844)

Author: thom

interface/web/admin/form/system_config.tform.php

@@ -595,12 +595,6 @@
 			'default' => 'http://www.ispconfig.org/atom',
 			'value'  => ''
 		),
-		'monitor_key' => array (
-			'datatype' => 'VARCHAR',
-			'formtype' => 'TEXT',
-			'default' => '',
-			'value'  => ''
-		),
 		'tab_change_discard' => array (
 			'datatype' => 'VARCHAR',
 			'formtype' => 'CHECKBOX',