Discontinue ocamlbuild in OCaml extraction examples

[tahina-pro]

This is a follow-up to #2815: while F* itself is now built with dune, not all of its examples are. As proposed by @msprotz in https://github.com/FStarLang/karamel/pull/333#issuecomment-1429090137, we should remove support for deprecated ocamlbuild across Everest:

• [ ] Remove the examples/hello/multifile ocamlbuild rule
• [ ] Convert examples/sample_project to dune
• [ ] Update the "Executing Programs > Compiling to OCaml" section of the tutorial
• [ ] Remove the ocamlbuild rule in the ulib/gmake template