Remove license text from the files that are not essential.

Remove the license text from files inside docs, examples and selftest,
to keep the license text only in relevant parts of the project.

References:

* https://fedoraproject.org/wiki/Packaging:LicensingGuidelines
* https://www.gnu.org/licenses/gpl-howto.html

Signed-off-by: Rudá Moura <rmoura@redhat.com>