overview: Always return a value in _onDragMotion()

Drag monitor functions are supposed to return a value, but
_onDragMotion() does not always do so. Add the missing return
value and remove unnecessary else.

js/ui/overview.js

@@ -207,8 +207,8 @@ Overview.prototype = {
 
         if (targetIsWindow &&
             dragEvent.targetActor._delegate.metaWindow == this._lastHoveredWindow)
-            return;
-        else
+            return DND.DragMotionResult.CONTINUE;
+
         this._lastHoveredWindow = null;
 
         if (this._windowSwitchTimeoutId != 0) {