ASoC: Intel: sst: Delete sst_shim_regs64; saved regs are never used

In commit 9a075265 ("ASoC: Intel: sst: Remove unused function
sst_restore_shim64()"), we deleted the sst_restore_shim64() since it
was never used.  ...but a quick look at the code shows that we should
also be able to remove the sst_save_shim64() function and the
structure members we were storing data in.

Once we delete sst_save_shim64() there are no longer any users of the
'sst_shim_regs64' structure.  That means we can delete it completely
and also avoid allocating memory for it.  This saves a whopping 136
bytes of devm allocated memory.  We also get the nice benefit of
avoiding an error path in the init code.

Note that the saving code that we're removing (and the comments
talking about how important it is to do the save) has been around
since commit 336cfbb0 ("ASoC: Intel: mrfld- add ACPI module").

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>