[node-webstreams] Remove unused PagesApi matching condition when setting react-server (#81037)
This looked like we were using `react-server` for Pages Router API routes. That wouldn't match Webpack behavior. We have an existing test in `test/e2e/import-conditions/import-conditions.test.ts` that checks the used import conditions in Pages Router API rotues.
Probably just an oversight when Turbopack started using `react-server` for Middleware and Instrumentation: https://github.com/vercel/next.js/pull/62134/files#diff-611c2f629b9a4c9ccb2fa2f6def7b7bbe8cde6fec2b5e085c3132250f7819803R164-R171
The aliasing implementation is now more closer structured to its Webpack counterpart in `packages/next/src/build/create-compiler-aliases.ts`.
I plan to move them closer together in a follow-up so that future alias work is easier to port between Webpack and Turbopack.