spi: Remove spi_flash_probe_fdt

Commit ba457562

 ("dm: x86: spi: Convert ICH SPI driver to driver model")
removed the last usage of the spi_flash_probe_fdt function, rendering it
obsolete.

This patch removes the function.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Signed-off-by: Mario Six <mario.six@gdsys.cc>