stepper_enable: Move motor_off() logic to stepper_enable.py

Directly disable all the stepper motors on a global motor_off() from
the StepperEnable() class in stepper_enable.py.  This simplifies the
toolhead and kinematic classes.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/kinematics/none.py

@@ -15,8 +15,6 @@ class NoneKinematics:
         pass
     def home(self, homing_state):
         pass
-    def motor_off(self, print_time):
-        pass
     def check_move(self, move):
         pass
     def get_status(self):