Remove residual references to subject objects

At some point we had subjects stored as objects, like:

```json
{
"key" : "/works/OL167326W",
"subjects" : [{"key" : "/subjects/Large_type_books"},{"key" : "/subjects/Fiction"},{"key" : "/subjects/Women"},{"key" : "/subjects/Women_lawyers"}],
}
```

No records like this exist any more, so the code no longer has to worry about it. I think some of the solr updating code checks for this; it can be updated (and possibly others).

The issue names the Solr updating code but no specific file or test; begin by searching that code for checks handling subject objects, then inspect any other references found. Done means the obsolete object-form subject handling is removed wherever the search identifies it.

Written by the indexing model from the issue text.