ASoC: imx: Remove unused Kconfig SND_MXC_SOC_SSI entry

SND_MXC_SOC_SSI looks to be unused, so kill it.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>