Disable FDW completely until all issues are ironed out

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EDIT: replaced by gitlab-org/gitlab-ee!4020

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What does this MR do?

Avoids FDW from being used, even if it was configured correctly on the database.

Why was this MR needed?

We still encounter a number of issues that indicate it's not production-ready:

• https://gitlab.com/gitlab-org/gitlab-ee/issues/4522 and
• https://gitlab.com/gitlab-org/gitlab-ee/issues/4491#note_53883782

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
• Internationalization required/considered
• [-] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

See gitlab-org/gitlab-ee#4558