[elixir] Fix warning of unused variable
[elixir] Simplify ListOps.append, avoiding overflow

Thanks @lancehalvorsen and @AaronLasseigne for the lead[1]!

[1]: http://exercism.io/submissions/0bae490632c9b9f3d5a21d2c

Submission: http://exercism.io/submissions/389de03e516b43056213eb61