NGINX: Remove unused substitutions module.

[elizabeth-dev]

What this PR does / why we need it:

As part of the migration to NJS and sunsetting of Lua scripts, we can remove the substitutions module, since it appears unused.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

Part of #12383

How Has This Been Tested?

The codebase has been inspected in order to ensure the module isn't used anywhere, and the e2e test suite has been passed.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: elizabeth-dev
Once this PR has been reviewed and has the lgtm label, please assign rikatz for approval. For more information see the Kubernetes Code Review Process.

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[k8s-ci-robot]

Hi @elizabeth-dev. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[strongjz]

/kind cleanup
/triage accepted
/ok-to-test