drop check_type_string, use upstream merged code instead
    
    Summary:
    The code from `check_type_string` was effectively merged into
    os-autoinst's `testapi::type_string` as an optional argument,
    so let's drop this downstream version and just have the 'safe
    typing' functions use `type_string`.
    
    Test Plan:
    Run tests, check they pass and work the same (i.e.
    make sure they're actually checking for screen change when
    typing).
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1033