Partly revert "make sure screen numbers are consecutive"

This partly reverts commit 074e4efe.
That patch for screen consistency was quite error prone: when a screen gets removed, only the mappings of the current activity will be removed, therefore the assumption that the screens on itemsOnDisabledScreens have all an id major than the ones in screenMapping does not hold. at every plasma startup therefore itemsOnDisabledScreens would get ids reassigned to numbers that won't make sense
    
BUG:464873