licensing: remove copyright statements from code

Keeping these up-to-date is a pain, and it's strictly speaking not
required. So let's just drop it, the git repository contains much
more accurate author-information.
licensing: do not track contributors

Tracking indevidual contributors in the source-code is a fool's
errand, and the code-repository already does this much more
accurately.

Replace individual names with "Contributors" to avoid having to
keep track. We were already missing alot of people anyway.