conf/layer.conf: Remove spi3 to spi6 overlays from revpi-core-3
conf/layer.conf: Remove spi3 to spi6 overlays from revpi-core-3

We have added at customer request the device tree overlays for the
spi buses 3 to 6. But revpi-core-3 is running an older kernel which
does not have these overlays.

Changelog-entry: Remove spi3 to spi6 overlays for revpi-core-3 because it runs an older kernel which does not have them
Signed-off-by: Florin Sarbu <3a837a37a2ac9072a088641ec330e0e5d5b86f4e@balena.io>

resin-os/resin-raspberrypi,resin-os/resin-raspberrypi