Drop `region` (`region_key` and `instance_key` are enough)

Motivation

The region metadata field was introduced at the storage format level to be able to tell which elements a table is annotating, without the need to examine the data in the table.

In memory the region metadata is redundant, as region can be computed on-the-fly looking at

adata = sdata.tables['my_table']
region_key = adata.uns['spatialdata_attrs']['region_key']
region = adata.obs[region_key].cat.categories

(the above assumes that the region_key column is categorical, which is already a requirement for certain plotting functions and operations (such as rasterize_bins())).

Dropping region will simplify the syntax when dealing with tables. For instance now when calling set_table_annotates_spatialelement(), the user needs to adjust the region metadata manually.

Plan

With this issue we propose (@melonora and I) to:

• drop the region metadata in-memory
• when writing, compute the metadata as above
• when reading, ignoring the region metadata
• provide a property computed as above for convenience (since the above code is verbose).

Start by tracing table metadata through the in-memory model and its read/write paths, including set_table_annotates_spatialelement(). Check how region_key, instance_key, and region are currently handled. Done means region is absent in memory, computed when writing, ignored when reading, and exposed through a convenience property.