SystemServer: Unbind to remove connection leak

Bug: 337569641
Fix: 337569641
Flag: Exempt, no logical change, cleaning no longer used variable
Test: manual |
      Check the current connectionRecord to Bluetooth
      adb shell dumpsys activity processes | grep ConnectionRecord | grep AdapterService
      kill bluetooth a couple time:
      adb shell am force-stop com.android.bluetooth; do it x times
      Check again the current connection number, should be still 1 and no DEAD connection
Change-Id: I9de7af00c889ba80f7c93413877019cbd948ad8c