vyos.configdict: T3972: bugfix QinQ vif-c removal triggered KeyError
vyos.configdict: T3972: bugfix QinQ vif-c removal triggered KeyError

Generic get_removed_vlans() function replaced the entire config dict when any
QinQ vif-c subinterface was deleted.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x