Eliminate multiple settings to extruder_duplication_enabled in G28

If the IDEX mode was previously set (via GCode or the LCD Panel) to a duplication mode...   It was lost after a G28 due to this redundant setting of 'extruder_duplication_enabled'. 

It is very important to the various use cases of IDEX machines that extruder_duplication_enabled is preserved across a G28.   The reason is this allows a person to use the LCD Panel to put the machine into a duplication mode, and then send a 'normal' part file to the printer and get two parts out of the process.

Marlin/src/gcode/calibrate/G28.cpp

@@ -461,8 +461,6 @@ void GcodeSuite::G28(const bool always_home_all) {
       delayed_move_time = 0;
       active_extruder_parked = true;
       extruder_duplication_enabled = IDEX_saved_duplication_state;
-      extruder_duplication_enabled = false;
-
       dual_x_carriage_mode         = IDEX_saved_mode;
       stepper.set_directions();