Update exercise template for luhn.

[jkrukoff]

This updates the exercise source template to match the test version in
the current test suite. In addition, it removes a function that is not
used in the current test suite.

My guess is that the template was not updated the last time the test suite was?

[juhlig]

Thanks. As far as I can see, there never was a test for create/1 at all. This was implemented before my time, so I don't know how it ended up in there.

But while you're at it, can you please have a look at #278, which actually obsoletes version tracking? I had been starting work on this once, but got distracted =^^=

In a nutshell, remove the version test from the test suite and the module stub, and the corresponding paragraph from README.md (that last thing, a run of bin/configlet generate . -o luhn can do that for you automatically).

[juhlig]

Your PR prompted me to take the improved test generator for a spin with luhn. Worked out quite well, I must say. Therefore, I have a PR that supersedes this one: #320.

Thus, I will close this PR, and thank you for your work.