don't show NO_PUBKEY warning if repo is signed by another key

don't show NO_PUBKEY warning if repo is signed by another key

Daniel Kahn Gillmor highlights in the bugreport that security isn't
improving by having the user import additional keys – especially as
importing keys securely is hard.

The bugreport was initially about dropping the warning to a notice, but
in given the previously mentioned observation and the fact that we
weren't printing a warning (or a notice) for expired or revoked keys
providing a signature we drop it completely as the code to display a
message if this was the only key is in another path – and is considered
critical.

Closes: 618445
(Backported from commit fb7b11ebb852fa255053ecab605bc9cfe9de0603)