Remove residual references to subject_people objects

Some `subject_people` fields used to have the form `{ type: "/type/text", value: String }` instead of just plain strings, like:

```json
{
"key" : "/works/OL55497W",
"subject_people" : [
{
"type" : "/type/text",
"value" : "Anton Pavlovich Chekhov (1860-1904)"
}
]
}
```

This is no longer the case, so any code checks for this (or worse, introduces it) can be removed.