drop perl Email::Valid dep on RHEL (#2166718)
    
    The git send-email command uses Email::Valid to check addresses.  If
    Email::Valid is not present, it falls back to a more basic regex match
    (which is not nearly as thorough as the checks Email::Valid performs).
    
    While Fedora (and EPEL 7/8 provide perl-Email-Valid, RHEL does not and
    does not wish to add the dependency.  Make it easier for RHEL to fork &
    sync from us by making the dependency conditional.
    
    References:
    https://bugzilla.redhat.com/2020487
    https://bugzilla.redhat.com/2046203
    http://public-inbox.org/git/20220620004427.3586240-1-trawets@amazon.com/T/#u
    4414f61 (add more git-email perl dependencies, 2021-11-13)