ASoC: Intel: sst: Remove unused function sst_restore_shim64()

Looks like the function has never been used since it was added by commit
b0d94acd ("ASoC: Intel: mrfld - add shim save restore"). Removing it
fixes the following warning when building with clang:

sound/soc/intel/atom/sst/sst.c:360:20: error: unused function
    'sst_restore_shim64' [-Werror,-Wunused-function]

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>