HFP: Remove dead code in native interface

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: Ifa0d3cbdf4487465782144b84a2bed5e4f7acc4e