Use `to_legacy_anndata` function from `spatialdata_io` instead of custom code

[fasterius]

Description of feature

We are currently using custom code as a work-around as a replacement for the to_legacy_anndata function from spatialdata_io, which did not exist when the work with SpatialData in this pipeline was initiated. This function is now merged in the SpatialData repo, but there is an issue with using it: https://github.com/scverse/spatialdata-io/issues/134.

As soon as this (and other possible issues) is fixed we should adopt the existing function instead of the custom one we have now.