Part of IOUApi should be left unimplemented in sync with training materials

[ooharawork]

In Module 10 of https://github.com/corda/corda-training-materials ,
the trainee or the reader is requested to implement getIOUs() and issueIOU() for practice,
however they are already implemented or the whole solutions are just commented out in corda-training-template.

For the training materials to be effective, we need modifications either to the code or to the document, such that the trainee can implement by his/her own.