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

[buhanec]

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)