OCaml documentation: remove reference to useDune2

doc/languages-frameworks/ocaml.section.md

@@ -45,10 +45,7 @@ Here is a simple package example.
 
 - It also accepts a `duneVersion` parameter (valid values are `"2"`, and
   `"3"`). The recommended practice is to set it only if you don't want the
-  default value and/or it depends on something else like package version. You
-  might see a not-supported argument `useDune2`. The behavior was `useDune2 =
-  true;` => `duneVersion = "2";` and `useDune2 = false;` => `duneVersion =
-  "1";`. It was used at the time when dune3 didn't exist.
+  default value and/or it depends on something else like package version.
 
 - It sets the optional `doCheck` attribute such that tests will be run with
   `dune runtest -p angstrom` after the build (`dune build -p angstrom`) is