beetsPackages.extrafiles: remove broken/unmaintained plugin

As noted in a comment in the package expression, the third-party `extrafiles`
plugin has not been maintained since 2020 and has been broken since the release
of beets version 2.0.0 in May 2024.

Since the `beets-stable` package has been updated to version 2.0.0, it is no
longer feasible to keep `beetsPackages.extrafiles` around.

Author: astratagem

pkgs/tools/audio/beets/default.nix

@@ -55,5 +55,6 @@ lib.makeExtensible (self: {
 
   alternatives = callPackage ./plugins/alternatives.nix { beets = self.beets-minimal; };
   copyartifacts = callPackage ./plugins/copyartifacts.nix { beets = self.beets-minimal; };
-  extrafiles = callPackage ./plugins/extrafiles.nix { beets = self.beets-minimal; };
+
+  extrafiles = throw "extrafiles is unmaintained since 2020 and broken since beets 2.0.0";
 })