MDL-53985 assign: Do not remove annotations

When an assignment submission is reverted to a draft, the timemodified on
the submission should not be modified. Changing the timemodified causes the
editpdf code to assume the pdf is stale, and forces all annotations to be removed.

mod/assign/locallib.php

@@ -7633,7 +7633,7 @@ class assign {
             return false;
         }
         $submission->status = ASSIGN_SUBMISSION_STATUS_DRAFT;
-        $this->update_submission($submission, $userid, true, $this->get_instance()->teamsubmission);
+        $this->update_submission($submission, $userid, false, $this->get_instance()->teamsubmission);
 
         // Give each submission plugin a chance to process the reverting to draft.
         $plugins = $this->get_submission_plugins();