Remove expired AddTrust_External_Root.crt because it breaks software

The AddTrust_External_Root.crt certificate has expired:

Data:
        Version: 3 (0x2)
        Serial Number: 1 (0x1)
    Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=SE, O=AddTrust AB, OU=AddTrust External TTP Network, CN=AddTrust External CA Root
        Validity
            Not Before: May 30 10:48:38 2000 GMT
            Not After : May 30 10:48:38 2020 GMT
        Subject: C=SE, O=AddTrust AB, OU=AddTrust External TTP Network, CN=AddTrust External CA Root
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:

This causes various client-side errors on Ubuntu 16.04 machines, about SSL certificate expiration, using (lib)curl for instance. Ubuntu 18.04 and up seem OK.

Removing 'mozilla/AddTrust_External_Root.crt' from /etc/ca-certificates.conf and running 'update-ca-certificates -f -v' helps. I'm not sure if removing it is universally the best solution, but I can't find any other bug reports about this on Launchpad, and this seems the quickest way to fix all clients.