Remove define which triggers a QMK bug

If SELECT_SOFT_SERIAL_SPEED is defined, it
triggers a QMK bug. A JSON with an invalid
type is generated. The validation against
keyboard.jsonschema fails. We can remove
SELECT_SOFT_SERIAL_SPEED because it's set
to the default value, anyway.

See qmk/qmk_firmware#26003

Author: jakoschiko

config.h

@@ -65,7 +65,6 @@
 #define SERIAL_USART_RX_PIN A10
 #define SERIAL_USART_CONFIG {115200, 3, 0, 0, 0};
 #define SERIAL_USART_FULL_DUPLEX
-#define SELECT_SOFT_SERIAL_SPEED 1
 #define SERIAL_DEBUG
 
 #define SPLIT_TRANSACTION_IDS_USER USER_SYNC_MMS    //multimode status