ASoC: intel: sst-haswell-ipc: Remove unused functions

Removes some functions that are not used anywhere:
sst_hsw_stream_unmute() sst_hsw_stream_mute() msg_set_stage_type()
sst_hsw_dx_get_state() sst_hsw_stream_set_write_position()
sst_hsw_stream_get_vol_reg() sst_hsw_stream_get_peak_reg()
sst_hsw_stream_get_pointer_reg() sst_hsw_stream_get_read_reg()
sst_hsw_stream_get_mixer_id() sst_hsw_stream_get_hw_id()
sst_hsw_mixer_set_volume_curve() sst_hsw_mixer_unmute()
sst_hsw_mixer_mute() sst_hsw_stream_set_volume_curve()

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Mark Brown <broonie@kernel.org>