Remove redundant `to_hash` call in `Need.list`.

As pointed out by @alext:
https://github.com/alphagov/maslow/commit/19004cf5aa9619905a3ff52744be15218a5a706e#commitcomment-5954731
Merge pull request #102 from alphagov/no-hash-call

Remove redundant `to_hash` call in `Need.list`.