Allow G26 with no heated bed (#13511)

Marlin/Configuration_adv.h

Marlin/src/gcode/bedlevel/G26.cpp

config/default/Configuration_adv.h

config/examples/3DFabXYZ/Migbot/Configuration_adv.h

config/examples/AlephObjects/TAZ4/Configuration_adv.h

config/examples/AliExpress/UM2pExt/Configuration_adv.h

config/examples/Anet/A2/Configuration_adv.h

config/examples/Anet/A2plus/Configuration_adv.h

config/examples/Anet/A6/Configuration_adv.h

config/examples/Anet/A8/Configuration_adv.h

config/examples/AnyCubic/i3/Configuration_adv.h

config/examples/ArmEd/Configuration_adv.h

config/examples/BIBO/TouchX/cyclops/Configuration_adv.h

config/examples/BIBO/TouchX/default/Configuration_adv.h

config/examples/BQ/Hephestos/Configuration_adv.h

config/examples/BQ/Hephestos_2/Configuration_adv.h

config/examples/BQ/WITBOX/Configuration_adv.h

config/examples/Cartesio/Configuration_adv.h

config/examples/Creality/CR-10/Configuration_adv.h

config/examples/Creality/CR-10S/Configuration_adv.h