Remove the revCount attribute from the generated flake registry

The revCount attribute is only present when the flake is an actual git
repo and not when using a flake reference with an input scheme for a git
forge that downloads tarballs instead (like github: or sourcehut:).
Because of this, the hash of the nixos config changes depending on
whether the config was built from a local git clone or from a flake ref
using one of those input schemes.
This leads to confusion (I lost quite some time figuring out what
happened here) and unnecessary rebuilds.
To my knowledge, there is no immediate benefit to including revCount in
the flake registry entry.

nixos/modules/config/nix-flakes.nix

@@ -72,7 +72,7 @@ in
                   type = "path";
                   path = config.flake.outPath;
                 } // filterAttrs
-                  (n: _: n == "lastModified" || n == "rev" || n == "revCount" || n == "narHash")
+                  (n: _: n == "lastModified" || n == "rev" || n == "narHash")
                   config.flake
               ));
             };