Removed global ca::TrackFit instance from ca::TripletConstructor.

This is a short one. I wanted to make sure that I understand this correctly.

It doesn't seem like the ca::TrackFit object needs to be persistent across different functions in the ca::TripletConstructor class. Also, ca::TrackFit doesn't seem to allocate much memory. So I propose to move it to local scope where it is used, to make things simpler.

The QA test doesn't show any problems.