spi: Kconfig: Remove old comments now that SPI slave is supported

Since commit 6c364062 ("spi: core: Add support for registering SPI
slave controllers") SPI slave is also supported, so remove the old
comments that say SPI slave is unsupported.
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/Kconfig

 #
 # SPI driver configuration
 #
-# NOTE:  the reason this doesn't show SPI slave support is mostly that
-# nobody's needed a slave side API yet.  The master-role API is not
-# fully appropriate there, so it'd need some thought to do well.
-#
 menuconfig SPI
 	bool "SPI support"
 	depends on HAS_IOMEM