timer.c: don't reset next_step_time when doing a step.

Doing so adds most of the interrupt execution time to the
step time, see line 126 of the same file:

    if (next_step_time == 0) {
      // new move, take current time as start value
      step_start = TCNT1;
    }

timer.c

@@ -69,7 +69,6 @@ void timer1_compa_isr() {
 ISR(TIMER1_COMPA_vect) {
 	// Check if this is a real step, or just a next_step_time "overflow"
 	if (next_step_time < 65536) {
-		next_step_time = 0;
 		// step!
 		timer1_compa_isr();
 		return;