MDL-64996 analytics: Don't mark static model as untrained after clearing

Static predictions models (i.e. those using a target based on
assumptions, not facts) are always considered as trained. Clearing them
must not mark them as untrained. Doing so would make them being skipped
by the prediction scheduled task.

analytics/classes/model.php

@@ -1626,7 +1626,10 @@ class model {
         // 1 db read per context.
         $this->purge_insights_cache();
 
-        $this->model->trained = 0;
+        if (!$this->is_static()) {
+            $this->model->trained = 0;
+        }
+
         $this->model->timemodified = time();
         $this->model->usermodified = $USER->id;
         $DB->update_record('analytics_models', $this->model);