Gitignore schema.json

[motin]

The schema JSON is generated from the schema YAML file. Thus, we can safely gitignore the JSON file and thus reduce risk. We originally kept both for diffs to clearly show how changes in the yaml affected the json, but do we still need both?
(Issue initiated by https://github.com/mozilla/shield-studies-addon-utils/pull/263/files#r216424894)
@gregglind ?

[motin]

Closing as wontfix since schema.json is the file that is verbatim copied into the study add-on, and keeping it in versioning is thus relevant.