activator: remove package from package-set

the typesafe activator is about to reach end of life at the end of this
month (2017-05) and the servers will be decommissioned at the ond of 2017.

See http://www.lightbend.com/community/core-tools/activator-and-sbt for
and the project's readme
(https://github.com/typesafehub/activator/blob/master/README.md) for further reference.