perf: replace delete with = undefined in request hot paths

packages/next/src/server/app-render/strip-flight-headers.ts

@@ -9,6 +9,6 @@ import { FLIGHT_HEADERS } from '../../client/components/app-router-headers'
  */
 export function stripFlightHeaders(headers: IncomingHttpHeaders) {
   for (const header of FLIGHT_HEADERS) {
-    delete headers[header]
+    headers[header] = undefined
   }
 }

packages/next/src/server/base-server.ts

@@ -2018,7 +2018,7 @@ export default abstract class Server<
     if (!addedNextUrlToVary) {
       // Remove `Next-URL` from the request headers we determined it wasn't necessary to include in the Vary header.
       // This is to avoid any dependency on the `Next-URL` header being present when preparing the response.
-      delete req.headers[NEXT_URL]
+      req.headers[NEXT_URL] = undefined
     }
   }
 

packages/next/src/server/request-meta.ts

@@ -372,7 +372,7 @@ export function removeRequestMeta<K extends keyof RequestMeta>(
   key: K
 ) {
   const meta = getRequestMeta(request)
-  delete meta[key]
+  meta[key] = undefined as RequestMeta[K]
   return setRequestMeta(request, meta)
 }