(CPR-52, CPR-65) Remove unnecessary apt key instruction for the nightly repos

The nightly key has been added to the puppetlabs-release package, so
specific instructions are no longer necessary.
Merge pull request #374 from haus/maint/master/remove_apt-key_instructions_for_nightly_repos

(CPR-52, CPR-65) Remove unnecessary apt key instruction for the nightly repos