[AudioFlinger] remove unused per stream volume/mute getter API

Bug: 317212590
Test: build & boot
Test: atest audiosystem_tests
Flag: EXEMPT removing dead code

Change-Id: I472da0eddc066c9694cbc9f1865c093d8c032415
Signed-off-by: François Gaffie <francois.gaffie@renault.com>
9 files changed