Remove the need to define the endstop pin for gear_stepper

[eamars]

I'm not sure the original intention and purpose of defining the endstop_pin for the gear_stepper. As of today (klipper 0.10.526), a slight modification to the code can remove the requirement to define the non-exist endstop_pin by

Replacing
self.gear_stepper.rail.steppers[0]

with
self.gear_stepper.rail

Pleaset let me know if that's a good idea. Otherwise I can create a PR for this change.

[eamars]

Just realized there is already an PR raised. #100