Fix a bogus UNIV_SYNC_DEBUG failure in the fix of Bug #59641

or Oracle Bug #11766513.

trx_undo_free_prepared(): Do not acquire or release trx->rseg->mutex.
This code is invoked in the single-threaded part of shutdown, therefore
a mutex is not needed.

storage/innobase/trx/trx0undo.c

@@ -1985,8 +1985,6 @@ trx_undo_free_prepared(
 /*===================*/
 	trx_t*	trx)	/*!< in/out: PREPARED transaction */
 {
-	mutex_enter(&trx->rseg->mutex);
-
 	ut_ad(srv_shutdown_state == SRV_SHUTDOWN_EXIT_THREADS);
 
 	if (trx->update_undo) {
@@ -2001,6 +1999,5 @@ trx_undo_free_prepared(
 			       trx->insert_undo);
 		trx_undo_mem_free(trx->insert_undo);
 	}
-	mutex_exit(&trx->rseg->mutex);
 }
 #endif /* !UNIV_HOTBACKUP */