ASoC: Remove unused function declaration in imx-ssi.h

These functions are removed in commit f0fba2ad
"ASoC: multi-component - ASoC Multi-Component Support".
Let's remove the leftover function declaration in header file.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>