device-xiaomi-nabu: Remove subpackages and hexagonrpcd to simplify the package (MR 6317)

As recommended on matrix, there is no need to keep everything in subpackages,
so they are being removed. Also, hexagonrpcd is being removed as it
doesn't work on this device’s soc yet.

Part-of: postmarketOS/pmaports!6317

[ci:skip-build]: already built successfully in CI