Ignore JSON files generated from PO files

What does this MR do?

It removes JS files containing translations from the repo.

Omnibus has been updated to generate these files when building the package: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1666

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We don't need those files in the repo given they are automatically generated from the PO files through the rake gettext:compile script, having those files on the repo generates conflicts most of the time.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?