OLD_SLOWDOWN does nothing, so remove it

Marlin/planner.cpp

@@ -929,15 +929,10 @@ void Planner::_buffer_line(const float &a, const float &b, const float &c, const
   int moves_queued = movesplanned();
 
   // Slow down when the buffer starts to empty, rather than wait at the corner for a buffer refill
-  #if ENABLED(OLD_SLOWDOWN) || ENABLED(SLOWDOWN)
-    bool mq = moves_queued > 1 && moves_queued < (BLOCK_BUFFER_SIZE) / 2;
-    #if ENABLED(OLD_SLOWDOWN)
-      if (mq) fr_mm_s *= 2.0 * moves_queued / (BLOCK_BUFFER_SIZE);
-    #endif
   #if ENABLED(SLOWDOWN)
-      //  segment time im micro seconds
+    // Segment time im micro seconds
     unsigned long segment_time = lround(1000000.0 / inverse_mm_s);
-      if (mq) {
+    if (moves_queued > 1 && moves_queued < (BLOCK_BUFFER_SIZE) / 2) {
       if (segment_time < min_segment_time) {
         // buffer is draining, add extra time.  The amount of time added increases if the buffer is still emptied more.
         inverse_mm_s = 1000000.0 / (segment_time + lround(2 * (min_segment_time - segment_time) / moves_queued));
@@ -947,7 +942,6 @@ void Planner::_buffer_line(const float &a, const float &b, const float &c, const
       }
     }
   #endif
-  #endif
 
   block->nominal_speed = block->millimeters * inverse_mm_s; // (mm/sec) Always > 0
   block->nominal_rate = ceil(block->step_event_count * inverse_mm_s); // (step/sec) Always > 0