Remove dead PPRRuntime task paths and getRuntimeStrategyForWalk
Pure refactor, no behavior change.
No prefetch task is ever scheduled with the PPRRuntime strategy: Link, Form,
router.prefetch and the link instances only create PPR or Full tasks, and a
runtime prefetch is chosen per walk inside the Partial Prefetching branch. So
the PPRRuntime arm of the legacy runtime-request dispatch, and its case in
diffSegmentAgainstCurrent, can never run. Drop PPRRuntime from
PrefetchTaskFetchStrategy and remove both.
getRuntimeStrategyForWalk had a single caller, so inline it into
wouldRuntimeRequestProvideMore.
