Do not map the branch to a release in the nix CI!

Author: mattam82

.nix/coq-overlays/metarocq/default.nix

@@ -13,7 +13,6 @@ let
   owner = "MetaRocq";
   defaultVersion = lib.switch coq.coq-version [
     { case = "9.0"; out = "1.4-9.0"; }
-    { case = "9.1"; out = "1.4-9.1"; }
   ] null;
   release = {
     "1.4-9.0".sha256 = "sha256-5QecDAMkvgfDPZ7/jDfnOgcE+Eb1LTAozP7nz6nkuxg=";