Remove BEEPER_PIN override in pins_RIGIDBOARD.h

Resolves #5926. This override is no longer needed since the RAMPS LCD controller pins were fixed.

Marlin/pins_RIGIDBOARD.h

 
 #elif ENABLED(REPRAP_DISCOUNT_SMART_CONTROLLER)
 
-  #undef BEEPER_PIN
-  #define BEEPER_PIN -1
-
   #undef  SD_DETECT_PIN
   #define SD_DETECT_PIN 22