Useless line removed (#9814)

nextAdvanceISR is set in the next if structure in every possible
situation, so it's useless to set it once more before.

Marlin/src/module/stepper.cpp

   // Timer interrupt for E. e_steps is set in the main routine;
 
   void Stepper::advance_isr() {
-    nextAdvanceISR = eISR_Rate;
 
     #if ENABLED(MK2_MULTIPLEXER)
       // Even-numbered steppers are reversed