contrib/gtk-tree-watch.pl: Remove bogus default socket path

Long description:

The default i3 socket path was set to "/tmp/nestedcons". This isn't
going to work for most people.

To apply this patch, use:
curl http://cr.i3wm.org/patch/157/raw.patch | git am

@@ -19,7 +19,7 @@ $window->signal_connect('delete_event' => sub { Gtk2->main_quit; });

 my $tree_store = Gtk2::TreeStore->new(qw/Glib::String/, qw/Glib::String/, qw/Glib::String/, qw/Glib::String/, qw/Glib::String/, qw/Glib::String/, qw/Glib::String/, qw/Glib::String/);

-my $i3 = i3("/tmp/nestedcons");

+my $i3 = i3();

 my $tree_view = Gtk2::TreeView->new($tree_store);