Remove tracking interfaces

[arokem]

This should supersede #1038, and removes the tracking.interfaces module altogether. It's been in limbo since as far back as 2012: 4ccf0bb

[Garyfallidis]

Thx @arokem This was an orphan for many years. And there is no plan to support traits. It would be good to have a thumbs up from @MrBago who wrote this code. So we can move ahead with the removal.
@arokem please also remove the dipy_tracking_gui command https://github.com/nipy/dipy/blob/master/bin/dipy_tracking_gui

[arokem]

Merged a commit from @MrBago removing a couple more outdated modules, and removed the tracking gui script.

[MrBago]

See #1056.