thunderbird-bin: remove libgnome dependency

It is deprecated and has been removed ages ago:

https://bugzilla.mozilla.org/show_bug.cgi?id=694570

It persists in the crash reporter for some reason but it is optional there:

https://github.com/mozilla/gecko-dev/blob/f66f5a235e7d74c29b951316f73001126a056734/toolkit/crashreporter/client/crashreporter_linux.cpp#L366-L370

pkgs/applications/networking/mailreaders/thunderbird-bin/default.nix

@@ -28,8 +28,6 @@
 , libXt
 , libxcb
 , libcanberra
-, libgnome
-, libgnomeui
 , gnome3
 , libGLU, libGL
 , nspr
@@ -108,8 +106,6 @@ stdenv.mkDerivation {
       libXt
       libxcb
       libcanberra
-      libgnome
-      libgnomeui
       libGLU libGL
       nspr
       nss