AP_HAL_ChibiOS: disable UART stats based on GCS being enabled

[peterbarker]

We may need to revisit this if we are able to transfer these files over
some other mechanism into the future

Suggested by @magicrub

[peterbarker]

Unsurprisingly saves a few bytes on f103-GPS

Binary Name    Text [B]         Data [B]     BSS (B)      Total Flash Change [B] (%)      Flash Free After PR (B)
-------------  ---------------  -----------  -----------  ----------------------------  -------------------------
ap_periph      -328 (-0.3241%)  0 (0.0000%)  0 (0.0000%)  -328 (-0.3236%) 

[magicrub]

I don't remember suggesting this but it looks good nonetheless