Fix switch desktop on screenedge while resizing a Wayland window

Summary:
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.

Reviewers: KWin, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6264