Remove GINIFile

[dopplershift]

Once #2635 is merged, we have two supported ways of reading GINI data:

1. GiniFile (and xarray support by passing that instance to open_dataset)
2. The xarray v2 backend support that can directly open the file

Since xarray is our primary data model, we should drop (1) (this is obviously an API break). At some point we should formally deprecate the use of GiniFile (for the small population that might be using archived GINI files).

When removing, we can substantially refactor the GINI support. At this time, all product information available previously in the direct reader (e.g. things in the prod_dec attribute) should be made available as attributes. We can also consider making the data field lazy loaded (though that's tricky due to the zlib frame-based compression in the products, so maybe not).