Fix undici warning in Node.js 18 (#42477)

- Remove warn for undici in Node.js 18
- Add node about the type being wrong

Follow-up to #42444. The other day I noticed that the type is incorrect.
This just removes the warning.

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## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Co-authored-by: JJ Kasper <jj@jjsweb.site>

packages/next/server/config.ts

@@ -57,17 +57,15 @@ const experimentalWarning = execOnce(
   }
 )
 
-export function setHttpClientAndAgentOptions(config: NextConfig) {
+export function setHttpClientAndAgentOptions(config: {
+  httpAgentOptions?: NextConfig['httpAgentOptions']
+  experimental?: {
+    enableUndici?: boolean
+  }
+}) {
   if (isAboveNodejs16) {
-    if (
-      config.experimental?.enableUndici &&
-      !config.experimental?.appDir &&
-      isAboveNodejs18
-    ) {
-      Log.warn(
-        `\`enableUndici\` option is unnecessary in Node.js v${NODE_18_VERSION} or greater.`
-      )
-    } else {
+    // Node.js 18 has undici built-in.
+    if (config.experimental?.enableUndici && !isAboveNodejs18) {
       // When appDir is enabled undici is the default because of Response.clone() issues in node-fetch
       ;(global as any).__NEXT_USE_UNDICI = config.experimental?.enableUndici
     }