Simplify Toplevel::isOnDesktop() and Toplevel::isOnAllDesktops()

Wayland- and X11-specific code paths are not needed.

If the window is on all virtual desktops, Toplevel::desktops() will be
an empty list.

If the window is on a particular virtual desktop, Toplevel::desktops()
will contain that desktop. On X11, the desktops list can have at most
only one item.