BT: Remove dead code in BluetoothHeadset

The following hidden API methods are not implemented by the stack and
should be removed:
 - getBatteryUsageHint()
 - acceptIncomingConnect()
 - rejectIncomingConnect()

The following hidden API methods are no longer needed due to lack of
usage and should be removed:
 - enableWBS()
 - disableWBS()
 - bindResponse()

Bug: 70336991
Test: make
Change-Id: I75bd6735573e4cbc61b5bef8722b8badac2e9170