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]

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.

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[strongjz]

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

[rikatz]

/lgtm
/cc @strongjz

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: elizabeth-dev, rikatz
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