libtorrent-rasterbar-1_2_x: use python3 by default

All packages that depend on it are already overriding the default python
to python3 in all-packages.nix. Let's just make it the default instead.

pkgs/applications/networking/p2p/tribler/default.nix

···
 }:
 
 let
-  libtorrent = (python3.pkgs.toPythonModule (
-    libtorrent-rasterbar-1_2_x.override { python = python3; })).python;
+  libtorrent = (python3.pkgs.toPythonModule (libtorrent-rasterbar-1_2_x)).python;
 in
 stdenv.mkDerivation rec {
   pname = "tribler";

pkgs/top-level/all-packages.nix

···
   ddrutility = callPackage ../tools/system/ddrutility { };
 
   inherit (callPackages ../applications/networking/p2p/deluge {
-    libtorrent-rasterbar = libtorrent-rasterbar-1_2_x.override { python = python3; };
+    libtorrent-rasterbar = libtorrent-rasterbar-1_2_x;
   })
     deluge-gtk
     deluged
···
 
   libtorrent-rasterbar-1_2_x = callPackage ../development/libraries/libtorrent-rasterbar/1.2.nix {
     inherit (darwin.apple_sdk.frameworks) SystemConfiguration;
-    python = python2;
+    python = python3;
   };
 
   libtorrent-rasterbar = libtorrent-rasterbar-2_0_x;