spi: Remove unused function spi_busnum_to_master()

The last user is gone since commit

	2962db71 ("staging/fbtft: Remove fbtft_device")

in 2019.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211007121415.2401638-3-u.kleine-koenig@pengutronix.de


Signed-off-by: Mark Brown <broonie@kernel.org>