csound: build with fluidsynth 2.x

csound has supported FluidSynth 2 since 6.13.0:
https://github.com/csound/csound/pull/1106

Since fluidsynth_1 has finally been broken by glib 2.70.0, so now is a
good opportunity to get rid of this dependency.

pkgs/applications/audio/csound/default.nix

···
 
 stdenv.mkDerivation rec {
   pname = "csound";
-  # When updating, please check if https://github.com/csound/csound/issues/1078
-  # has been fixed in the new version so we can use the normal fluidsynth
-  # version and remove fluidsynth 1.x from nixpkgs again.
   version = "6.16.2";
 
   hardeningDisable = [ "format" ];

pkgs/top-level/all-packages.nix

···
 
   csa = callPackage ../applications/audio/csa { };
 
-  csound = callPackage ../applications/audio/csound {
-    fluidsynth = fluidsynth_1;
-  };
+  csound = callPackage ../applications/audio/csound { };
 
   csound-manual = callPackage ../applications/audio/csound/csound-manual {
     python = python27;