Remove docfx, which is no longer being used

[cgillum]

Static code analysis is flagging some JavaScript code that's present in our docfx directory. We're not actually using docfx for anything (we migrated all our content to learn.microsoft.com many years ago), so we can safely remove this code.

Note that GitHub Pages is already disabled on this repo, so there really shouldn't be anything that's using this content.

We may need a similar change to the main branch.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation PR is ready to merge and referenced in pending_docs.md
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests) [N/A]
• My changes do not require any extra work to be leveraged by OutOfProc SDKs
  • Otherwise: That work is being tracked here: #issue_or_pr_in_each_sdk
• My changes do not change the version of the WebJobs.Extensions.DurableTask package
  • Otherwise: major or minor version updates are reflected in /src/Worker.Extensions.DurableTask/AssemblyInfo.cs
• My changes do not add EventIds to our EventSource logs
  • Otherwise: Ensure the EventIds are within the supported range in our existing Windows infrastructure. You may validate this with a deployed app's telemetry. You may also extend the range by completing a PR such as this one.
• My changes should be added to v3.x branch.
  • Otherwise: This change only applies to Durable Functions v2.x and will not be merged to branch v3.x.