Remove TrackMate

This branch removes `src-plugins/TrackMate_` and related files from Fiji's Git repository. The TrackMate project now has its own [dedicated repository](https://github.com/fiji/TrackMate) in the `fiji` namespace. So TrackMate is still a core component of Fiji, it just does not live in `fiji.git` anymore. Such modularization of Fiji's core plugins is the direction we are going, so that it is easier to for contributors to fork and develop only those components of interest.