backports: don't hardcode an old gpg key for Ubuntu

[kenyon]

Summary

Same as #847 but for Ubuntu.

The gpg keys used to sign the Ubuntu repos, including backports, come with standard Ubuntu installations, so this Puppet module doesn't need to add any key.

Having this old key causes Puppet run failures for 20.04 and newer machines without Internet access, since it tries to fetch the key from the Internet.

Related Issues (if any)

847

[kenyon]

Not sure why that Ubuntu 18.04 test is failing, but it started failing in #1124, and seems unrelated to this module's code.

[bastelfreak]

@kenyon let me know if bug label isn't correct.