Automatically set X2 stepper pins

[thinkyhead]

Background: For DUAL_X_CARRIAGE the X2 pins can be automatically set to the first unused Extruder stepper pins, so that's what this PR does (if the X2 pins are not already set in the configuration).

Since it's possible to combine Y_DUAL_STEPPER_DRIVERS or Z_DUAL_STEPPER_DRIVERS with a DUAL_X_CARRIAGE on a board with enough free stepper drivers, this PR moves those stepper pins to the next available E plug when DUAL_X_CARRIAGE is enabled.

The automated results end up as:

Configuration	Steppers Order
Normal	X, Y, Z, E0
Dual Extruder	X, Y, Z, E0, E1
Dual X Carriage	X, Y, Z, E0, E1, X2
Dual Y Steppers	X, Y, Z, E0, Y2
Dual Z Steppers	X, Y, Z, E0, Z2
Dual X Carriage + Dual Y Steppers	X, Y, Z, E0, E1, X2, Y2
Dual X Carriage + Dual Z Steppers	X, Y, Z, E0, E1, X2, Z2

• This PR also removes the default/example overrides for the X2 stepper pins from Configuration_adv.h.

[nivekhan]

In the case that Dual X, Dual Y, AND Dual Z are implemented, would it make sense to add in pins.h
#define Y2_Z2_E_INDEX INCREMENT(EXTRUDERS)
(line 357) right after line 368 so that the order goes X,Y,Z,E0,E1,X2,Y2,Z2?

[thinkyhead]

@caenan0 Funny you should mention that…. #4271