Removed UKFTractography extension

This is motivated by the fact the extension prevent the windows factory from
completing the extension build. See https://github.com/pnlbwh/ukftractography/issues/3

Additionally, the extension description file that should be checked in
should be named "UKFTractography.s4ext"
Merge branch 'update-UKFTractography'

* update-UKFTractography:
  Removed UKFTractography extension