MDL-37897 Remove useless Blackboard question import format

lib/pluginlib.php

@@ -362,6 +362,7 @@ class plugin_manager {
     public static function is_deleted_standard_plugin($type, $name) {
         static $plugins = array(
             // do not add 1.9-2.2 plugin removals here
+            'qformat' => array('blackboard'),
         );
 
         if (!isset($plugins[$type])) {
@@ -508,7 +509,7 @@ class plugin_manager {
             ),
 
             'qformat' => array(
-                'aiken', 'blackboard', 'blackboard_six', 'examview', 'gift',
+                'aiken', 'blackboard_six', 'examview', 'gift',
                 'learnwise', 'missingword', 'multianswer', 'webct',
                 'xhtml', 'xml'
             ),