Allow SERVO0_PIN override on Creality Melzi (#21007)

Marlin/src/pins/sanguino/pins_MELZI_CREALITY.h

 #define LCD_PINS_D4                           30  // ST9720 CLK
 
 #if ENABLED(BLTOUCH)
-  #define SERVO0_PIN                          27
-  #undef BEEPER_PIN
+  #ifndef SERVO0_PIN
+    #define SERVO0_PIN                        27
+  #endif
+  #if SERVO0_PIN == BEEPER_PIN
+    #undef BEEPER_PIN
+  #endif
 #elif ENABLED(FILAMENT_RUNOUT_SENSOR)
   #ifndef FIL_RUNOUT_PIN
     #define FIL_RUNOUT_PIN                    27