spi: remove unused adi_spi3.h header

include/linux/spi/adi_spi3.h is unused since commit 47838669 ("spi: remove blackfin related host drivers")
Finish the cleaning by removing it.
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>