Listing apt keys doesn't work after apt-file deprecation

[TheRealFalcon]

The code to list GPG keys expects to receive a keyring. It calls

gpg --no-options --with-fingerprint --no-default-keyring --list-keys --keyring <key_file>

That's fine when using the deprecated /etc/apt/trusted.gpg as trusted.gpg is a key ring.

However, its only call site will also list all files under /etc/apt/trusted.gpg.d, which are not keyrings. They are simply dearmored public keys and so will not work with the --list-keys command.

That said, it appears that this code isn't even used anywhere, so we might be better off just deleting it altogether.