HFP: Remove dead code in HFP profile

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: I84c5555879597857a45f370d8de82d67cbd4e379