overlays: Delete spi0-hw-cs

The spi0-hw-cs overlay is unnecessary (and actually harmful) with the
current kernels. Delete it, leaving a note in the README and a
deprecation message from the firmware:

    dterror: overlay 'spi0-hw-cs' is deprecated: no longer necessary

Signed-off-by: Phil Elwell <phil@raspberrypi.com>