=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
1) Works as expected, it'll blow up if it didn't throw that, and not blow up if it did. Not sure what you mean (assertionerror extends throwable)?
2) It's kind of a specialised method for the tck - used specifically for rule 3.9 - why'd someone expect assertion error?