Remove additional license files for the CSS2 test suite

In https://github.com/web-platform-tests/rfcs/blob/master/rfcs/simplify_license.md
we resolved to simplify the license to the plain 3-Clause BSD License,
with the support of the W3C, and this was implemented in
#11191.

Since then, contributions have been under that plain 3-Clause BSD
License, per our CONTRIBUTING.md.

Remove these additional license files to make it clear that not all
content of css/CSS2/ can be used under those licenses.

Author: foolip