kinematics: Convert get_rails() method to get_steppers()

All callers of get_rails() actually just want the steppers, so return
them directly.

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

klippy/kinematics/delta.py

@@ -80,8 +80,8 @@ class DeltaKinematics:
             % (math.sqrt(self.max_xy2), math.sqrt(self.slow_xy2),
                math.sqrt(self.very_slow_xy2)))
         self.set_position([0., 0., 0.], ())
-    def get_rails(self, flags=""):
-        return list(self.rails)
+    def get_steppers(self, flags=""):
+        return [s for rail in self.rails for s in rail.get_steppers()]
     def _actuator_to_cartesian(self, spos):
         sphere_coords = [(t[0], t[1], sp) for t, sp in zip(self.towers, spos)]
         return mathutil.trilateration(sphere_coords, self.arm2)