MuonTrackSteeringTools: Fix non-reentrancy in MuPatHitTool.

Scott Snyderrequested to merge
ssnyder/athena:thread.MuonTrackSteeringTools-20201210 into master

Move the handling of garbage out of MuPatHitTool to its callers; they were already managing lifetimes correctly. A few other cleanups as well.

Eventually, the way ownership is handled here should be rationalized, but one step at a time.

See ATLASRECTS-5813

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