Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/modules/migrate/migrate.install

@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Contains install and update functions for Migrate.
+ */
+
+/**
+ * @addtogroup updates-8.0.0-beta
+ * @{
+ */
+
+/**
+ * Remove load plugin references from existing migrations.
+ */
+function migrate_update_8001() {
+  $config_factory = \Drupal::configFactory();
+  foreach ($config_factory->listAll('migrate.migration.') as $migration_config_name) {
+    $migration = $config_factory->getEditable($migration_config_name);
+    $migration->clear('load');
+    $migration->save(TRUE);
+  }
+}
+
+/**
+ * @} End of "addtogroup updates-8.0.0-beta".
+ */