drm/amd/display: remove audio mute control in hpo dp

VPG doesn't have the ability to mute audio output by sending all 0s in
audio SDP. The existing implemention is disabling audio SDP instead.
This is same as what dp_audio_enable does. Since it is no longer
referenced by any callers, we decided to remove this interface for
simplicity.

Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>