workspace: Avoid setting an undefined width

Since commit c04289853f the `width` parameter may be undefined, but
it is still set unconditionally, whoops.

js/ui/workspace.js

@@ -566,7 +566,6 @@ var WindowOverlay = new Lang.Class({
     _animateOverlayActor(actor, x, y, width, height) {
         let params = { x: x,
                        y: y,
-                       width: width,
                        time: Overview.ANIMATION_TIME,
                        transition: 'easeOutQuad' };