function removed from libre in https://github.com/baresip/re/pull/1407

Index: src/dtls.c
--- src/dtls.c.orig
+++ src/dtls.c
@@ -52,7 +52,6 @@ static void conn_destructor(void *arg)
 
 	list_unlink(&conn->le);
 	tmr_cancel(&conn->tmr);
-	dtls_set_handlers(conn->tlsc, NULL, NULL, NULL, NULL);
 	mem_deref(conn->tlsc);
 }
 
