stepper: Replace PrinterHomingStepper with PrinterRail

Update the code to use the term "rail" when dealing with a motor
controlled "axis".  A rail has a series of steppers and endstops that
control that motor controlled "axis".

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

klippy/extras/z_tilt.py

@@ -32,8 +32,7 @@ class ZTilt:
     def handle_connect(self):
         kin = self.printer.lookup_object('toolhead').get_kinematics()
         try:
-            z_stepper = kin.get_steppers('Z')[0]
-            z_steppers = [z_stepper] + z_stepper.extras
+            z_steppers = kin.get_rails('Z')[0].get_steppers()
         except:
             logging.exception("z_tilt stepper lookup")
             raise self.printer.config_error(