Remove the old estimation OSGi setting (compaction.gainThreshold)

Currently, there are two implementations for finding out the gain in repository size after running compaction: the old one, CompactionGainEstimate and the new one, SizeDeltaGcEstimation. Similarly, there are also two configurations for customising them, in SegmentNodeStoreService, compaction.gainThreshold and compaction.sizeDeltaEstimation.

At the moment both of them are exposed as OSGi configurations, but only the new one should be exposed (e.g. compaction.sizeDeltaEstimation).

It must be evaluated whether it makes sense to keep the logic associated with the old implementation.