Remove duplicate task `bundle exec rake gettext:po_to_json` from CI

What does this MR do?

The rake task gettext:po_to_json is called within the task gitlab:assets:compile, so unless I'm missing something, it shouldn't need to be called before it. This MR removes a couple lines where this was happening, which should save a few seconds on every pipeline. 🎉

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

Why was this MR needed?

It appears the gettext:po_to_json task was added to gitlab-ci.yml, and then a month later it was also added to the gitlab:assets:compile rake task.

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?