MDL-55565 core_backup: Fix warning when restoring question categories

Fixed a call to get_records_sql, which was being used incorrectly in
restore_find_best_target_context to determine a unique contextid.

backup/util/dbops/restore_dbops.class.php

@@ -793,7 +793,7 @@ abstract class restore_dbops {
                      // Prepare the query
                      list($stamp_sql, $stamp_params) = $DB->get_in_or_equal($stamps);
                      list($context_sql, $context_params) = $DB->get_in_or_equal($contexts);
-                     $sql = "SELECT contextid
+                     $sql = "SELECT DISTINCT contextid
                                FROM {question_categories}
                               WHERE stamp $stamp_sql
                                 AND contextid $context_sql";