generate_user_config_files is creating spurious masking entries in the config files

`generate_user_config_files` is creating spurious (and possibly erroneous?) masking entries.

```
[masking]
stream_ap5_cice-U = -1:None:None,180:None:None
stream_ap5_grid-V = -1:None:None,180:None:None
stream_ap6_cice-U = -1:None:None,180:None:None
stream_ap6_grid-V = -1:None:None,180:None:None
stream_onm_cice-U = -1:None:None,180:None:None
stream_onm_grid-V = -1:None:None,180:None:None
```

Looks like the problem is in the `get_masking_attributes` function.

https://github.com/MetOffice/CDDS/blob/574dd77aa358d6d8eadbb6e617547d830353308f/cdds/cdds/configure/user_config.py#L215-L225

Start in cdds/configure/user_config.py at get_masking_attributes around lines 215-225, then trace how generate_user_config_files consumes its result. Reproduce the generated config and compare the listed masking entries with the source attributes; done means spurious entries such as stream_ap5_cice-U and stream_onm_grid-V are no longer generated.

Written by the indexing model from the issue text.