Simplify X11Window::removeTransient()

If a transient parent is removed, X11Window::removeTransient() will try
to upgrade the transient to a group transient.

On the other hand, that code is broken because Window::removeTransient()
will unset transientFor() so we never hit the code path in the if
statement.

While we could fix it, I think it's not worth it because transient
handling code is already mental and making group transients and normal
transients disjoint will help to keep the things simpler.