Failure opening stats file

I got this failure:

```console
gen_open_stats_file
FATAL ERROR
/home/users/david.davies/cylc-run/mo-bundle-415/log/job/1/ctest_opsinputs__spice_nag/01/job.stats: cannot open: I/O error on unit 7: No such file or directory
```

This is transient, but I suppose the question here is do we need stats files? Can we just remove this functionality? Looking at the code this isn't actually failing to write job.stats, it is failing to write a file called fort.7, which looks like it was a hack to cover up some other issue.