Remove extra MAX_FREQ_TIME define (PR#148)

Marlin/planner.cpp

 
   // Max segement time in us.
   #ifdef XY_FREQUENCY_LIMIT
-    #define MAX_FREQ_TIME (1000000.0 / XY_FREQUENCY_LIMIT)
 
     // Check and limit the xy direction change frequency
     unsigned char direction_change = block->direction_bits ^ old_direction_bits;