fix the ambiguous import error in TestDecisionTree class.

Multiple classes are imported twice by including the line `import cc.factorie.app.classify._` as well as `import cc.factorie.app.classify.backend._`, causing compilation errors like the following:

```
reference to C45DecisionTreeTrainer is ambiguous;
it is imported twice in the same scope by
import cc.factorie.app.classify.backend._
and import cc.factorie.app.classify._
Error occurred in an application involving default arguments.
      new DecisionTreeMulticlassTrainer(new C45DecisionTreeTrainer))
```