chore: delete the definition of pin A3 (#9798)

Co-authored-by: Lucas Saavedra Vaz <[email protected]>

Author: Lesords

variants/XIAO_ESP32C3/pins_arduino.h

@@ -17,7 +17,6 @@ static const uint8_t SCK = 8;
 static const uint8_t A0 = 2;
 static const uint8_t A1 = 3;
 static const uint8_t A2 = 4;
-static const uint8_t A3 = 5;
 
 static const uint8_t D0 = 2;
 static const uint8_t D1 = 3;