angband: remove unused SDL2_sound dependency

`angband` referrs to SDL2_sound in debug outputs of
configure script/cmake, but actually depends on
SDL2_mixer instead.

[1] https://github.com/angband/angband/blob/f3797fe9acb712fb9c7ba43d2f779bd18eaf2ada/configure.ac#L699

pkgs/by-name/an/angband/package.nix

@@ -7,7 +7,6 @@
   enableSdl2 ? false,
   SDL2,
   SDL2_image,
-  SDL2_sound,
   SDL2_mixer,
   SDL2_ttf,
 }:
@@ -29,7 +28,6 @@ stdenv.mkDerivation (finalAttrs: {
     ++ lib.optionals enableSdl2 [
       SDL2
       SDL2_image
-      SDL2_sound
       SDL2_mixer
       SDL2_ttf
     ];