xfce.xfce4-pulseaudio-plugin: Drop automakeAddFlags

We don't use it anywhere, the substituteInPlace already does the job.

pkgs/desktops/xfce/panel-plugins/xfce4-pulseaudio-plugin/default.nix

···
 { lib
 , mkXfceDerivation
-, automakeAddFlags
 , exo
 , gtk3
 , libcanberra
···
   pname = "xfce4-pulseaudio-plugin";
   version = "0.4.8";
   sha256 = "sha256-7vcjARm0O+/hVNFzOpxcgAnqD+wRNg5/eqXLcq4t/iU=";
-
-  nativeBuildInputs = [
-    automakeAddFlags
-  ];
 
   postPatch = ''
     substituteInPlace configure.ac.in --replace gio-2.0 gio-unix-2.0