ModelAdmin2.search_fields assigned twice.

`ModelAdmin2.search_fields` assigned twice. Is it really necessary?

``` python
# ...
search_fields = ()
# ...

search_fields = []
```

I've removed the line (types.py):

``` python
search_fields = []
```

Ran the tests, and I get no error.

The duplicate assignment is in types.py; start by inspecting both ModelAdmin2.search_fields declarations and run the existing test suite. Done means the redundant assignment is removed without changing behavior and the tests still pass.

Written by the indexing model from the issue text.