The file-picker for repositories that use "fp-login-form" uses the classes label and input on the table cells that hold the labels/inputs.

This is tricky to track down in order to remove them. The template is here:

https://github.com/moodle/moodle/blob/master/files/renderer.php#L906-L908

but I'm guessing if I change those classes then the template will no longer work. The function isn't called directly from anywhere, instead some meta-programming gathers up the results of all the functions with a similar name. It's possible the rest of the work is done in javascript rather than PHP.