The documentation for the option says...
> If you want a default binding to be passed through to the website,
> bind it to null.
but if you actually try to set a key to `null`, it causes an error.
> A definition for option
> `programs.qutebrowser.keyBindings."<Ctrl+Shift+Tab>".normal' is not
> of type `strings concatenated with " ;; "'.
So this commit implements unbinding as it is documented.
096d9c04b3