lv2: drop GTK 2 dependency

gtk2 is optional and only used for some example plug-ins. In particular,
removing `gtk2` disables building UI module for `$out/lib/lv2/eg-sampler.lv2`
(`<http://lv2plug.in/plugins/eg-sampler#ui>`) and the whole
`$out/lib/lv2/eg-scope.lv2` (`<http://lv2plug.in/plugins/eg-scope>`).

pkgs/development/libraries/audio/lv2/default.nix

@@ -1,7 +1,6 @@
 { stdenv
 , lib
 , fetchurl
-, gtk2
 , libsndfile
 , pkg-config
 , python3
@@ -23,7 +22,6 @@ stdenv.mkDerivation rec {
   ];
 
   buildInputs = [
-    gtk2
     libsndfile
     python3
   ];