DB: drop UserInfo table
    
    We no longer need the `UserInfo` table. Bugzilla credentials are not checked at
    the moment (and even if we start to use them again, we'll probably don't need to
    validate them), and API keys are not used at all.
    
    This also solves a DeprecationWarning that the `crypt` module (used inside
    `UserInfo`) will be removed from Python soon.