I guess you have to use jQuery for it or edit all the files. There's no way to do it with CSS only.
Either you do:
$(document).ready(function() {
$('a[target="_blank"]').prop("target", "_self");
});
Or:
$(document).ready(function() {
$('a[target="_blank"]').removeAttr('target');
});
Not sure if there're other ways around it. Probably.
Best regards.