Remove un-implememented singleton export

[timleslie]

Marking this as a patch change, as this export is undocumented and should not have been used.

[changeset-bot]

🦋 Changeset detected

Latest commit: c7a9847

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@keystone-next/keystone	Patch
@keystone-next/test-utils-legacy	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/keystonejs/keystone-next-docs/GjiZ9wkwZucoWVcLdXiCj3RDitAJ
✅ Preview: https://keystone-next-docs-git-remove-singleton-export-keystonejs.vercel.app

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit c7a9847:

Sandbox	Source
@keystone-next/example-sandbox	Configuration