Removed __traits_version__ from json files

Created by: jjenthought

The workflow is a dictionary which contains lists of dictionaries, so only the traits_version keys in the top level dict were being removed. This changed the function so it should do it recursively. It seems fine but the only test is a on a single hardcoded example at the minute. Refs. issue https://github.com/force-h2020/force-bdss/issues/134

Fixes #134 (closed)