=tck remove not used assertion method which could lead to NPE

Removes not used `expectException` method, we only ever expect exceptions to validate their contents.

The former impl could lead to NPE as `flop()` does not throw, so the `return null` was indeed reachable (it should not have - while implementing this method I must have been under the impression that flop would throw).

This does not change semantics of the TCK, just a bugfix.
Refs https://github.com/reactive-streams/reactive-streams/issues/155