Remove twentysixteen exception from nightly-builds script

We’d like to bring Twenty Sixteen to trunk, alongside all other default themes.
For that the nightly build script in /usr/local/bin/make-nightly.sh would need to be adjusted not pull in the theme from GitHubGitHub GitHub is a website that offers online implementation of git repositories that can can easily be shared, copied and modified by other developers. Public repositories are free to host, private repositories require a paid subscription. GitHub introduced the concept of the ‘pull request’ where code changes done in branches by contributors can be reviewed and discussed before being merged be the repository owner. https://github.com/ anymore. [dotorg13183] removed that from the release script already.

It would be great if it could be adjusted before the next planned release, RC2 on June 1st. Thank you!

#prio2