Revert "tests: devicetree: test supported devices API"

This reverts commit 4c32e21f with some
manual conflict resolution.

It's not clear that the supported devices are being properly computed,
so let's revert this for v2.7.0 until we've had more time to think
it through.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>