ATLASRECTS-5614: Fix GlobalChi2Fitter reference parameter cloning memory leak

This commit fixes a minor mistake in !35388 (merged) that may lead to a small but nonetheless important memory leak. During the move away from the old memory management inside the fitter, I accidentally added an extra and unnecessary clone of a track's reference parameters which would result in small memory leak. This commit gets rids of this vestige of the old memory model by modifying the getters, setters, and internal representation of a track's reference parameters and replacing them by the standard smart pointer ownership semantics. The extraneous clone is removed as a direct result of this.

Related to ATLASRECTS-5614.