RecordingActivityMonitor: remove spam from death events

Release death handler when its recorder is removed from tracking
to avoid logspam of "Unexpected event" messages.

Bug: 136279123
Test: atest android.media.cts.AudioRecordingConfigurationTest
      adb shell logcat | grep "Unexpected event"

Change-Id: I085ee545a07c7d3ada05c575878998a0e76a135b