Remove `isRoutePPREnabled` from render options and delete dead PPR branch (#90713)
Stacked on https://github.com/vercel/next.js/pull/90704
This PR replaces `experimental.isRoutePPREnabled` render options with
`cacheComponents` and removes unused code.
Verified deployment tests pass on CI:
https://github.com/vercel/next.js/actions/runs/22536405880