Remove deprecated use of structlog's wrap_dict

We aren't calling a logger's `new()`, `bind()` or `unbind()` methods,
so assuming this is unused.

It was originally copied over from cohort-extractor, for which the code
was initially added a while ago.

The current way of doing things if this is needed is to use
`contextvars`:

```python
structlog.contextvars.bind_contextvars()
structlog.contextvars.unbind_contextvars()
```

https://www.structlog.org/en/22.3.0/thread-local.html
https://www.structlog.org/en/22.3.0/contextvars.html

databuilder/utils/log_utils.py

@@ -21,7 +21,6 @@
         structlog.processors.UnicodeDecoder(),
         structlog.stdlib.ProcessorFormatter.wrap_for_formatter,
     ],
-    context_class=structlog.threadlocal.wrap_dict(dict),
     logger_factory=structlog.stdlib.LoggerFactory(),
     wrapper_class=structlog.stdlib.BoundLogger,
     cache_logger_on_first_use=True,