Remove SoundTriggerMiddleware death notification

The death notifier for the middleware service is unnecessary
- We don't use mService elsewhere (nor check its nullity)
- We re-query the service upon each call

Additionally, creating a new death notifier on each query is redundant,
and results in memory leaks (since each notifier takes state, and we
never unlink to death).

Test: Compiles
Bug: 237968209
Change-Id: Ib53ec65235d4284357e0f6649bb6f65b2de83b99