MKR Zero: Remove rainbowio to save some space

[stonehippo]

The MKR Zero has a pretty small internal flash, so reducing the size of the core CircuitPython build is ideal. Rainbowio has been removed from some other SAM21 boards without external flash, so doing that here, too.

[dhalbert]

We didn't remove this on MKR Zero before, because it wasn't necessary to get it to fit with all translations. We are not seeing build failures right now with this board without the removal. What necessitated this for you? Thanks.

[dhalbert]

We hope to turn rainbowio back on, on the boards where we turned it off, after some space saving happens. Turning it off was only for expediency to get the release out.

[stonehippo]

@dhalbert I was in the process of making a custom build (to fit audiobusio and audioio onto the MKR Zero for I2S) and I noticed that rainbowio had been removed from several of the other SAM21 boards. I've been using a couple of these boards for messing with some I2C stuff as well, and there hasn't been a lot of space for libraries.

That said, I can see why you'd want to get rainbowio back on all boards for baseline consistency in CP.

I'll close this one.

[dhalbert]

NP - In 8.0.0 we will be able to remove some things, such as OneWire being in both onewireio (new location) and busio (old location), and that should free up the space we need.