Remove `x-forwarded-fetch`

https://github.com/fedify-dev/fedify/pull/355#discussion_r2258738135
If you need it, you can use this instead of `fedifyWith`:
```ts
// middleware.ts
import { hasFederationAcceptHeader, integrateFederation } from "@fedify/next";
import { getXForwardedRequest } from "x-forwarded-fetch";

export default function middleware(request: Request) {
  if(hasFederationAcceptHeader(request) {
    integrateFederation(getXForwardedRequest(request))
  }
}
```