xdg-autostart-generator: ignore DBusActivatable=true

See https://wiki.gnome.org/HowDoI/DBusApplicationLaunching and
https://wiki.gnome.org/Initiatives/GnomeGoals/DBusActivatable for a description
of this key:
> Instead of the typical UNIX-style fork()/exec() approach to process creation,
> launching an application is done by sending a D-Bus message to the well-known
> name of that application, causing a D-Bus activation.
>
> Starting processes with D-Bus activation ensures that each application gets
> started in its own pristine environment, as a direct descendent of the
> session -- not in the environment of whatever its parent happened to be. This
> is important for ensuring the app ends up in the correct cgroup, for example.

So this motivation is not important for us: we launch stuff ourselves better.

This fixes warnings during boot:
systemd-xdg-autostart-generator[2274]: /etc/xdg/autostart/org.freedesktop.problems.applet.desktop:92: Unknown key name 'DBusActivatable' in section 'Desktop Entry', ignoring.