Update unreleased changelog fragments that later changes superseded
Several fragments no longer described the next release: the semantic diff no longer reads fragments (#8908), switches over many constants are no longer faster (#8900), the playground API v9 bump and its revert are both unreleased, two watcher fixes apply only to rescript-rust, and the breaking changes of the OCaml build system missed some user-visible differences listed in rewatch-ocaml/README.md.