net: caif: Remove unused caif SPI driver

While chasing in_interrupt() (ab)use in drivers it turned out that the
caif_spi driver has never been in use since the driver was merged 10 years
ago. There never was any matching code which provides a platform device.

The driver has not seen any update (asided of treewide changes and
cleanups) since 8 years and the maintainers vanished from the planet.

So analysing the potential contexts and the (in)correctness of
in_interrupt() usage is just a pointless exercise.

Remove the cruft.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>