ASoC: imx-ssi: Remove unused driver

The imx-ssi driver was only used by i.MX non-DT platforms.

Since 5.10-rc1, i.MX has been converted to a DT-only platform and all
board files are gone.

Remove the imx-ssi audio driver as there are no more users at all.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20201110203937.25684-6-festevam@gmail.com


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