fix: remove check for [.dune] files (#8611)

These files are absolutely ancient and there's nobody out there still
has them. We were supposed to get rid of this warning in 3.0, but
there's no reason to not do it now.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

doc/changes/8611.md

@@ -0,0 +1,2 @@
+- Remove warning against `.dune` files generated by pre dune 2.0 (#8611,
+  @rgrinberg)

src/dune_rules/findlib.ml

@@ -64,22 +64,6 @@ let hash { stdlib_dir; paths; builtins; ext_lib } =
 
 let findlib_predicates_set_by_dune = Ps.of_list [ P.ppx_driver; P.mt; P.mt_posix ]
 
-let check_dot_dune_exists ~dir ~dir_contents name =
-  match dir_contents with
-  | Error _ -> ()
-  | Ok fnames ->
-    let fname = sprintf "%s.dune" (Lib_name.to_string name) in
-    if List.mem fnames fname ~equal:String.equal
-    then
-      User_warning.emit
-        ~loc:(Loc.in_file (Path.relative dir fname))
-        [ Pp.text
-            ".dune files are ignored since 2.0. Reinstall the library with dune >= 2.0 \
-             to get rid of this warning and enable support for the subsystem this \
-             library provides."
-        ]
-;;
-
 let has_double_underscore s =
   let len = String.length s in
   len >= 2
@@ -98,7 +82,6 @@ let has_double_underscore s =
 let to_dune_library (t : Findlib.Package.t) ~dir_contents ~ext_lib =
   let loc = Loc.in_file t.meta_file in
   let add_loc x = loc, x in
-  check_dot_dune_exists ~dir:t.dir ~dir_contents t.name;
   let archives = Findlib.Package.archives t in
   let obj_dir = Obj_dir.make_external_no_private ~dir:t.dir in
   let modes : Lib_mode.Map.Set.t =