Remove "cast" from Promise test

[It looks like Promise.cast was killed a while ago](https://github.com/domenic/promises-unwrapping/issues/98), so no need to test for it.

[Jake did the same change in his Polyfill](https://github.com/jakearchibald/es6-promise/commit/baa0195a4453d87031e276ac35e5a1aa24a7bb18), so I think that this change should be safe.