sandbox: spi: Drop unused sandbox_spi_parse_spec function

Commit 1289e967 ("sandbox: spi: Drop command-line SPI option") dropped
support for specifying SPI devices on the command line, removing the only
user of sandbox_spi_parse_spec(). Remove the function too.

Fixes: 1289e967 ("sandbox: spi: Drop command-line SPI option")
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>