Remove parent_ssi field

[corylown]

From Slack discussion:

Both parent_ssi and parent_ssim are no longer used (as of arclight 1.x) and should be removed.
In arclight 0.x, both of those fields were used in context navigation & breadcrumbs. But as of 1.x, the indexing is smarter and uses more of Solr's built-in nested document indexing. And yes, the breadcrumbs now use parent_ids_ssim , a field introduced in 1.x.
The direct parent document id is now represented in nest_parent (the underscores are a tell that it's a Solr nested document convention). This ends up being used in queries that build out the hierarchy.