yarn2nix: remove GPLv3 licence

All contributors that have made contributions at all
likely to be copyrightable have agreed to relicense this
code under the same MIT licence as the rest of Nixpkgs in
<#334374>.

Note that this applies to all previous versions of the code, including
in older Nixpkgs versions where the GPLv3 text is still present and the
original yarn2nix repositories before they were imported into Nixpkgs.

Closes: #334267
Closes: #334374