AudioService: remove dead BT code

  Remove dead BT code that supported the implementation of
AudioManager.handleBluetoothA2dpActiveDeviceChange() but
was made obsolete with d7c9598a ("Restructure A2DP active device
change").

Bug: 132416679
Test: m -j (no functional changes)

Change-Id: Iedca8a5225548f205920ef50bb3ad8865eb99d1d