Remove leaf segment force-refetch check (#90836)
Remove the `parallelRoutesKeys.length === 0` condition from
`renderComponentsOnThisLevel` in walk-tree-with-flight-router-state.
This check was added to support the behavior where clicking the same
link twice forces all page segments to re-render on the server. This
should no longer be necessary because the client already adds a refetch
marker on those segments, so there's no need to special-case leaf
segments on the server side.
