License updates (dev_4_4) + obsolete code removal

This branch brings the copyrights and licenses of the entire source tree up to date, including the LICENSE.txt files and the copyright header at the top of each file. It also brings the license-maven-plugin configuration into the SCM, rather than having it be hosted on an external server, which makes future licensing updates much more straightforward.

It also removes some obsolete code (`legacy` and `native/bf-itk-*` subtrees), since they are unsupported now, and updating the licensing for those projects would be a waste of time.