Remove useless line from stepper ISR

The flag is already set inside planner.get_current_block(). Doing it
twice isn't making it better..

Marlin/stepper.cpp

@@ -343,7 +343,6 @@ void Stepper::isr() {
     // Anything in the buffer?
     current_block = planner.get_current_block();
     if (current_block) {
-      SBI(current_block->flag, BLOCK_BIT_BUSY);
       trapezoid_generator_reset();
 
       // Initialize Bresenham counters to 1/2 the ceiling