PostgreSQL 10 Clean Up After Restore
The files should go away on their own after a checkpoint or two (if there is little activity, it might take longer for the files to go away.  If they were renamed already in anticipation of being recycled back when the server was very active, they won't go away until they get re-used).  If you don't want it to use up to 1GB of WAL, then you should lower "max_wal_size".  Otherwise, it might do what you gave it permission to do.