Remove duplicate test for utils.import_string.

Removes duplicate test of:

```python

self.assert_is(utils.import_string(u'cgi.escape'), cgi.escape)
```

on https://github.com/tony/werkzeug/commit/c4429f245f9e51aefd1cc49ccac688b5d9f4ed16#diff-4af618d8be4053d1aaa0862448b1609cL179 and https://github.com/tony/werkzeug/commit/c4429f245f9e51aefd1cc49ccac688b5d9f4ed16#diff-4af618d8be4053d1aaa0862448b1609cL183.