Remove unused AVRCP hidden API

AudioManager is informed of absolute volume support
directly by AvrcpTargetService. This API has no known usage
and isn't implemented properly so we can remove it safely.

Having a binder for AVRCP isn't necessary for now as only
the setAvrcpAbsoluteVolume API is used. Other interactions
are made directly between the service and the framework.

Bug: 301242682
Tag: #feature
Test: atest BluetoothA2dpTest
Change-Id: I53edc9b61b17eed91578182381633daeb0e9d3df