chore: Remove unused environment variable (#32520)

## Description
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extremely technical (helps the content team)._
>
> _Please also include relevant motivation and context. List any
dependencies that are required for this change. Add links to Notion,
Figma or any other documents that might be relevant to the PR._

This PR removes unused environment variable

Fixes #`Issue Number`  
_or_  
Fixes `Issue URL`
> [!WARNING]  
> _If no issue exists, please create an issue first, and check with the
maintainers if the issue is valid._

## Automation

/ok-to-test tags="@tag.Sanity"

### 🔍 Cypress test results
<!-- This is an auto-generated comment: Cypress test results  -->
> [!IMPORTANT]  
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/8613707077>
> Commit: `f86cfafda68bc6281dc3f7f431f4756d5f0c61af`
> Cypress dashboard url: <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=8613707077&attempt=1"
target="_blank">Click here!</a>
> All cypress tests have passed 🎉🎉🎉

<!-- end of auto-generated comment: Cypress test results  -->

app/client/public/index.html

@@ -40,8 +40,6 @@
         const CLOUD_HOSTING = parseConfig("__APPSMITH_CLOUD_HOSTING__");
         const ZIPY_KEY = parseConfig("__APPSMITH_ZIPY_SDK_KEY__");
         const AIRGAPPED = parseConfig("__APPSMITH_AIRGAP_ENABLED__");
-        // NREUM variable is required when we are we are are dynamically loading new relic browser agent script
-        NREUM = undefined
       </script>
     <script>
         window.__APPSMITH_CHUNKS_TO_PRELOAD =