Reduce RuboCop offenses

You can check for offenses which is introduced after generation of `.rubocop_todo.yml`; with `rubocop`.

FYI, the `objects` argument in `Inch::Api::Suggest#all_filenames` is never used.
https://github.com/rrrene/inch/blob/12fd81d572d4131883543d64d79fb7656744eb0c/lib/inch/api/suggest.rb#L72