Screenedges only allows to switch desktop while moving window, not
while resizing. There was a special branch which only checked this for
X11 windows but not for Wayland windows.
This change removes a no longer needed cast from AbstractClient to Client
so that the check whether a window is getting resized works for both
X11 and Wayland clients. The cast was introduced in a time when
AbstractClient did not yet support the isResize method.