Remove mention of RNA (uracil) from nucleotide-count problem?

[kytrinyx]

The metadata for this problem has been simplified, and it might be worth changing the test suite and example so that theres no mention of RNA or uracil.

I've also changed the API of the problem in Ruby so that there is no error checking in the object itself, only in the factory that creates it: https://github.com/exercism/xruby/blob/master/nucleotide-count/example.rb#L2-L8

I don't know if this is relevant or idiomatic in Haskell.