stepper: Remove add_to_endstop() method

Now that the PrinterStepper and MCU_stepper classes have been merged,
there is no need for the add_to_endstop() method.

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

klippy/extras/probe.py

@@ -242,7 +242,7 @@ class ProbeEndstopWrapper:
     def _build_config(self):
         kin = self.printer.lookup_object('toolhead').get_kinematics()
         for stepper in kin.get_steppers('Z'):
-            stepper.add_to_endstop(self)
+            self.add_stepper(stepper)
     def home_prepare(self):
         toolhead = self.printer.lookup_object('toolhead')
         start_pos = toolhead.get_position()