metro_m0_express: put back audiobusio

this now fits locally (though it's very close to capacity);
let's see what the CI says.

```
make BOARD=metro_m0_express clean && make BOARD=metro_m0_express  TRANSLATION=ja
...
253448 bytes used, 248 bytes free in flash firmware space out of 253696 bytes (247.75kB).
```

Author: jepler

ports/atmel-samd/boards/metro_m0_express/mpconfigboard.mk

@@ -11,7 +11,6 @@ EXTERNAL_FLASH_DEVICE_COUNT = 2
 EXTERNAL_FLASH_DEVICES = "S25FL216K, GD25Q16C"
 LONGINT_IMPL = MPZ
 
-CIRCUITPY_AUDIOBUSIO = 0
 CIRCUITPY_BITBANGIO = 0
 CIRCUITPY_COUNTIO = 0
 CIRCUITPY_I2CPERIPHERAL = 0