Floss: Clean up IBluetoothGatt

Cleaning up some unused public APIs. The public APIs will be re-added in
the future for Scanning and GATT Server when designed with
go/floss-api-principles.

Bug: 224603540
Tag: #floss
Test: Manual - Build Floss on Linux

Change-Id: If35fd339bdeb116006189969eb839900c1f522b6