spi: Remove spi_setup_slave_fdt

A previous patch removed the spi_flash_probe_fdt function, which
contained the last call of the spi_setup_slave_fdt function, which is
now equally 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>