Correctly raise ValueError on unknown binding in KeyBindings.remove()

When trying to remove from KeyBindings by Keys, an undocumented UnboundLocalError exception is raised instead of the documented ValueError.

To repro can add something like the following to test_remove_bindings:

        h3 = handlers.controla

        ...

        # Test passing unbound key sequence to the remove() function
        with pytest.raises(ValueError):
            bindings.remove(h3)
        with pytest.raises(ValueError):
            bindings.remove(Keys.ControlA)

Start at KeyBindings.remove() and the existing test_remove_bindings test. Reproduce removal of an unbound handler and Keys value there, then verify the documented ValueError is raised for both cases. Done when the regression coverage passes and the prior UnboundLocalError no longer occurs.