make the dual z-endstop optional (PR#2277)

Marlin/Configuration_adv.h

Marlin/configurator/config/Configuration_adv.h

Marlin/example_configurations/Felix/Configuration_adv.h

Marlin/example_configurations/Hephestos/Configuration_adv.h

Marlin/example_configurations/K8200/Configuration_adv.h

Marlin/example_configurations/SCARA/Configuration_adv.h

Marlin/example_configurations/WITBOX/Configuration_adv.h

...n/example_configurations/delta/biv2.5/Configuration_adv.h

.../example_configurations/delta/generic/Configuration_adv.h

...mple_configurations/delta/kossel_mini/Configuration_adv.h

Marlin/example_configurations/makibox/Configuration_adv.h

.../example_configurations/tvrrug/Round2/Configuration_adv.h