Fixed plugins.feedback_plugin_send failure

Fixed assertion failure introduced along with MDEV-6150 (thd must be unlinked
before calling "delete thd").

plugin/feedback/sender_thread.cc

@@ -265,6 +265,7 @@ static void send_report(const char *when)
     thd->set_status_var_init();
     thread_count--;
     thd->killed= KILL_CONNECTION;
+    thd->unlink();
     mysql_cond_broadcast(&COND_thread_count);
     mysql_mutex_unlock(&LOCK_thread_count);
     delete thd;