Remove Elixir beer-song exercise

I've been getting feedback that people think it's annoying.
It also costs me more trouble than other exercises to check, probably
due to the high amount of "clutter" (long strings) and I don't really
see the lesson it tries to teach.
Merge pull request #1305 from pminten/remove-elixir-beer-song

Remove Elixir beer-song exercise