Remove mkrzero code

[Ymit24]

What?

Remove code from embedded_firmware which depends on features from the mkrzero that isn't present in the custom hardware. (E.g. LED_BUILTIN)

Why?

It is not useful to try and drive a pin which is not connected.

How?

• Remove all occurrences of code from the firmware which use these features.

[Ymit24]

Going to knock this out really quick.

[Ymit24]

Done. This one was very quick and easy.

[Ymit24]

Merged.