Remove unused class

It functionality is now all implemented in ExplicitOuter.
Clean up code relating to interfaces.

Previous implementation was confused about the meaning of interface (with default methods
or without?).

Now instead of Interface/JavaInterface we have

    PureInterface   - all members are abstract methods
    NoInits         - all members are methods
Merge pull request #219 from dotty-staging/cleanup/interface-rebased

Cleaning up interfaces.