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 GitHub 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