SPI flash layer: remove unused function spi_flash_free()

We don't ever free memory in coreboot, hence drop spi_flash_free() and
spi_free_slave()

Change-Id: I0ca3f78574ceb4516e7d33c06ab1a58abfb3b0ec
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1273
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/drivers/spi/spi_flash.c

@@ -297,12 +297,5 @@ err_manufacturer_probe:
 err_read_id:
 	spi_release_bus(spi);
 err_claim_bus:
-	spi_free_slave(spi);
 	return NULL;
 }
-
-void spi_flash_free(struct spi_flash *flash)
-{
-	spi_free_slave(flash->spi);
-	free(flash);
-}