Reduce code in gcode_M81 (PR#2497)

Marlin/Marlin_main.cpp

  */
 inline void gcode_M81() {
   disable_all_heaters();
-  st_synchronize();
-  disable_e0();
-  disable_e1();
-  disable_e2();
-  disable_e3();
   finishAndDisableSteppers();
   fanSpeed = 0;
   delay(1000); // Wait 1 second before switching off