mtd: spi-nor: remove unused read_xfer/write_xfer hooks

struct spi_nor_xfer_cfg and read_xfer/write_xfer hooks were never used by
any driver. Do some cleanup by removing them.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>