deprecated warning: remove dune alert as non fatal for compatibility

purpose (ocaml < 4.08.0) and add a warning removal in code

dune

 (executable
  (public_name opam-custom-install)
- (flags (:standard -w -24 -alert --deprecated))
+ (flags (:standard -w -24))
  (libraries opam-client))

opam-custom-install.ml

@@ -150,6 +150,7 @@ let custom_install cli =
           $ OpamArg.build_options cli
           $ no_recompilations $ packages $ cmd)
 
+[@@@ocaml.warning "-3"]
 let () =
   OpamStd.Option.iter OpamVersion.set_git OpamGitVersion.version;
   OpamSystem.init ();