Remove dead code from stepper.h

These values are always supplied by `Conditionals_post.h`.

Marlin/src/module/stepper.h

@@ -50,18 +50,6 @@
 // Estimate the amount of time the Stepper ISR will take to execute
 //
 
-#ifndef MINIMUM_STEPPER_PULSE
-  #define MINIMUM_STEPPER_PULSE 0UL
-#endif
-
-#ifndef MAXIMUM_STEPPER_RATE
-  #if MINIMUM_STEPPER_PULSE
-    #define MAXIMUM_STEPPER_RATE (1000000UL / (2UL * (unsigned long)(MINIMUM_STEPPER_PULSE)))
-  #else
-    #define MAXIMUM_STEPPER_RATE 500000UL
-  #endif
-#endif
-
 #ifdef CPU_32_BIT
 
   // The base ISR takes 792 cycles