meta/files/layers.schema.json: drop the layers property

meta/files/layers.schema.json: drop the layers property

This is a leftover from one of the RFC iterations, where
the property contained available machines, distros and templates.
As all of those were dropped from the final version, there is no
reason to list the layers either anymore.

Normally this would be a backwards incompatible change, but as
the layer setup itself was just merged, I think we can do a quick
fixup :-)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>