FTS doesn't actually seem to use the superuser's username for anything.
That allowed removing probeUser and retrieveUserAndDb(). All the remaining
calls to FtsFindSuperuser() were from cdbresynchronizechangetracking.c,
but all of those callers just discarded the return value.
FtsFindSuperuser() doesn't throw any errors, either, so it didn't serve
any error-checking purpose either. Remove it all.