Replace /etc/apt/grml.key with /etc/apt/trusted.gpg.d/grml-archive-keyring.gpg

/etc/apt/grml.key was a PGP public key block Public-Key (old) and
required installation via apt-key.

By instead placing a `GPG key public ring` into
/etc/apt/trusted.gpg.d/grml-archive-keyring.gpg we don't need
this extra apt-key invocation any longer, which might actually
fail with:

| E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation

Installation of gnupg isn't enough though, since then we run
into:

| 10:45:56 Warning: apt-key output should not be parsed (stdout is not a terminal)
| 10:45:58 gpg: can't connect to the agent: IPC connect call failed

Drop old /etc/grml/fai/config/files/etc/apt/grml.key/GRMLBASE via maintscript

Thanks: Antoine Beaupré <anarcat@debian.org> for a related PR
in https://github.com/grml/grml-debian-keyring/pull/3