servicemanager: rely on auto unlinkToDeath

unlinkToDeath happens automatically when the last strong ref to a proxy
goes away (see 80d23935).

Bug: 136027762
Test: boot and check for log:
    "onLastStrongRef automatically unlinking death recipients"

Change-Id: Iec16013548d9bda24f6e96d5cf80c69071953cb0