Update split keyboard docs (#4735)

* Remove unused I2C_MASTER_RIGHT setting

* Update documentation about split keyboard options

quantum/split_common/split_util.c

@@ -36,8 +36,7 @@ static void setup_handedness(void) {
     #ifdef EE_HANDS
       isLeftHand = eeprom_read_byte(EECONFIG_HANDEDNESS);
     #else
-      // I2C_MASTER_RIGHT is deprecated, use MASTER_RIGHT instead, since this works for both serial and i2c
-      #if defined(I2C_MASTER_RIGHT) || defined(MASTER_RIGHT)
+      #ifdef(MASTER_RIGHT)
         isLeftHand = !has_usb();
       #else
         isLeftHand = has_usb();