checks: remove darwinChanges

It seems like this has been broken since
26bab2fd32 from 2018; macOS `diff(1)`
does not support these arguments and the `$PATH` has been fixed since
that point. Before that, it would presumably only have worked if you
added GNU diffutils to `environment.systemPackages`.

Proactively notifying users about breaking changes isn’t a bad idea,
but the activation script doesn’t seem like the right place for it,
and as it’s non‐blocking it’s likely that even if it worked it
would get drowned out by the later output anyway. I’d like to move
the changelog into release notes as part of the manual, which would
requires changes to this anyway. Since it’s been broken for almost
seven years anyway, let’s just drop it for now and revisit later.

modules/system/checks.nix

@@ -8,22 +8,6 @@ let
 
   cfg = config.system.checks;
 
-  darwinChanges = ''
-    darwinChanges=/dev/null
-    if test -e /run/current-system/darwin-changes; then
-      darwinChanges=/run/current-system/darwin-changes
-    fi
-
-    darwinChanges=$(diff --changed-group-format='%>' --unchanged-group-format= /run/current-system/darwin-changes $systemConfig/darwin-changes 2> /dev/null) || true
-    if test -n "$darwinChanges"; then
-      echo >&2
-      echo "CHANGELOG" >&2
-      echo >&2
-      echo "$darwinChanges" >&2
-      echo >&2
-    fi
-  '';
-
   macOSVersion = ''
     IFS=. read -ra osVersion <<<"$(sw_vers --productVersion)"
     if (( osVersion[0] < 11 || (osVersion[0] == 11 && osVersion[1] < 3) )); then
@@ -368,7 +352,6 @@ in
   config = {
 
     system.checks.text = mkMerge [
-      darwinChanges
       (mkIf cfg.verifyMacOSVersion macOSVersion)
       (mkIf (cfg.verifyBuildUsers && !config.nix.configureBuildUsers) oldBuildUsers)
       (mkIf cfg.verifyBuildUsers buildUsers)