Fix licensing problems

These two commits fix the licenses problems referred in https://github.com/colobot/colobot/pull/241 :

* cbf4f75 drops the license statement for the public-domain image, which is useless
* 8c1bc2e replaces the CREATIVE-licensed thunder sound by one public-domain thunder sound which I found on archive.org

Thanks for considering, cheers,

OdyX