please remove the dependency on “conf-libev” (not available on Windows)

[bdodrem]

Hi,

Is it possible to remove the dependency on “conf-libev” in order to be able to build Opium on Windows-Cygwin (libev not available on Cygwin)?

Also, I don't think this dependency is needed for Lwt.

Thanks.

diff --git a/nix/opam-selection.nix b/nix/opam-selection.nix
index 544035b..aed7fe3 100644
--- a/nix/opam-selection.nix
+++ b/nix/opam-selection.nix
@@ -638,7 +638,6 @@ in
         dune-configurator dune
         cppo;
         ocaml-syntax-shims = selection.ocaml-syntax-shims or null;
-        conf-libev = selection.conf-libev or null;
         base-unix = selection.base-unix or null;
         base-threads = selection.base-threads or null;
       };