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.