tabbox: Drop desktop switching

Tabbox supports two operation modes: switching between windows and desktops. Switching between windows is more commonly used. Desktop switching is not exposed in user settings and it requires some advanced knowledge of kwin's internals to enable it.

On the other hand, over the past years, we've double downed on effects like desktop grid and overview to provide graphical means to switch between virtual desktops.

This change drops desktop switching because it's effectively unused to simplify the tabbox code, which can be very handy for the future refactorings of window switching.