remove unnecessary exec ctx prep from Iteratee.flatMap

commit 2b276d24 implemented the lines:
```
case Step.Done(a, Input.Empty) => executeIteratee(f(a))(ec /* still on same thread; let executeIteratee do preparation */ )
case Step.Done(a, e) => executeIteratee(f(a))(ec.prepare /* still on same thread; let executeIteratee do preparation */ ).pureFlatFold {
```
However, the `ec.prepare` is incongruous with the adjacent comment, and the preparation is definitely not required as the preparation is indeed handled by `executeIteratee`.
Merge pull request #2257 from dwhjames/topic/iteratee-ec-prepare

Remove unnecessary execution context preparation from flatMap on iteratees