apt-key: ignore any error produced by gpgconf --kill

gpgconf wasn't always equipped with a --kill option as highlighted by
our testcases failing on Travis and co as these use a much older version
of gpg2. As this is just for cleaning up slightly faster we ignore any
error a call might produce and carry on. Use a recent enough gpg2
version if you need the immediate killing…

Gbp-Dch: Ignore
Reported-By: Travis CI