remove trailing whitespace

[ee7]

With this PR, the following command produces no output when run in the repo root directory.

git grep -r '[[:blank:]]$'

The commits that added the whitespace:

• a71637f lint: add annotation for key features in config.json
• fea7bcc lint: allow overlap of files.example and files.exemplar values
• d0cd08c lint: error on missing concept/exercise files
• 3a55f57 lint: support error annotations