kinematics: Remove support for identifying Z steppers

The caller can now determine which steppers are connected to cartesian
Z movement via the new stepper.is_active_axis() method.  It is
therefore no longer necessary for the kinematic code to identify these
steppers.

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

klippy/kinematics/cartesian.py

@@ -49,9 +49,7 @@ class CartKinematics:
             self.printer.lookup_object('gcode').register_command(
                 'SET_DUAL_CARRIAGE', self.cmd_SET_DUAL_CARRIAGE,
                 desc=self.cmd_SET_DUAL_CARRIAGE_help)
-    def get_steppers(self, flags=""):
-        if flags == "Z":
-            return self.rails[2].get_steppers()
+    def get_steppers(self):
         rails = self.rails
         if self.dual_carriage_axis is not None:
             dca = self.dual_carriage_axis