remove skipping of certificate update in GenericCertificateResolver

[Keksoj]

As pointed out by @justinkb, @newdev8 and others in this issue: #774 , those lines in the add_certificate method of the GenericCertificateResolver prevent persisting several subdomains:

        // We do not need to update the entry, if the certificate is already registered
        if self.get_certificate(&fingerprint).is_some() {
            return Ok(fingerprint);
        }

Removing these lines do not break any test, as the CI on this Pull Request should prove, and according to @justinkb, does solve the issue.

The best would be to have an e2e test for this behaviour with several subdomains. What do you think @Wonshtrum ?