Ender 3 V2 DWIN cleanup (#21061)

Marlin/src/lcd/dwin/e3v2/dwin.h

 typedef struct {
   TERN_(HAS_HOTEND,     int16_t E_Temp    = 0);
   TERN_(HAS_HEATED_BED, int16_t Bed_Temp  = 0);
-  TERN_(HAS_PREHEAT,    int16_t Fan_speed = 0);
+  TERN_(HAS_FAN,        int16_t Fan_speed = 0);
   int16_t print_speed     = 100;
   float Max_Feedspeed     = 0;
   float Max_Acceleration  = 0;
   #if HAS_LEVELING
     bool leveling_offset_flag:1;
   #endif
-  #if HAS_FAN
-    AxisEnum feedspeed_axis;
-  #endif
-  AxisEnum acc_axis, jerk_axis, step_axis;
+  AxisEnum feedspeed_axis, acc_axis, jerk_axis, step_axis;
 } HMI_Flag_t;
 
 extern HMI_value_t HMI_ValueStruct;

Marlin/src/pins/stm32f1/pins_CREALITY_V4.h

 #define HEATER_0_PIN                        PA1   // HEATER1
 #define HEATER_BED_PIN                      PA2   // HOT BED
 
-#define FAN_PIN                             PA0   // FAN
-#define FAN_SOFT_PWM
+#ifndef FAN_PIN
+  #define FAN_PIN                           PA0   // FAN
+#endif
+#if PIN_EXISTS(FAN)
+  #define FAN_SOFT_PWM
+#endif
 
 //
 // SD Card