sandbox: spi: Remove an incorrect free()

We must not free data that is managed by driver mode. Remove this line,
which is a hangover from the pre-driver-model code.

This fixes a problem where 'sf probe' crashes U-Boot if the backing file
for the SPI flash cannot be found.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jteki@openedev.com>
Tested-by: Jagan Teki <jteki@openedev.com>
Reviewed-by: Tom Rini <trini@konsulko.com>