hap: Remove HAP features callback

The non-@SystemApi cannot be implemented by the app on error-prone build, but
has to be implemented in the regular build. To compile the app on both this
has to be removed.

Bug: 150670922
Tag: #feature
Test: atest CtsBluetoothTestCases:android.bluetooth.cts.BluetoothHapClientTest BluetoothInstrumentationTests
Sponsor: jpawlowski@
Change-Id: I7ee01d74726f39320f56b36895a1d80a26852014