Remove previous_millis_cmd update from extruder runout prevention code

This seems to defeat the purpose of previous_millis_cmd, preventing the
time elapsed from previous_millis_cmd from ever reaching
max_inactive_time or stepper_inactive_time while the heat was on.

Marlin/Marlin_main.cpp

      current_position[E_AXIS]=oldepos;
      destination[E_AXIS]=oldedes;
      plan_set_e_position(oldepos);
-     previous_millis_cmd=millis();
      st_synchronize();
      WRITE(E0_ENABLE_PIN,oldstatus);
     }