rna-transcription: Update based on changes to specification by removing some test cases

[Stargator]

exercism/problem-specifications#1027 Removes tests from the specification, now we can choose to keep them we don't have to be one-to-one with the specification. We can ignore some cases or use them all and then build off from there.

And exercism/problem-specifications#1028 is just a cleanup of the former. It doesn't impact the specification's implementation.

Personally, I would opt to keep the cases, but I want to throw it out to the @exercism/dart maintainers

[Stargator]

Closing due to a similar issue exists and has a wider scope to incorporate schema changes. #94