gtkgesture: Drop GdkWindow checks

gtkgesture: Drop GdkWindow checks

Those are now needless and wrong, as we get guarantees that handled
events will contain widget-relative coordinates. A side effect is
that these events are very possibly not explicitly sent to the
GdkWindow that implementations expect, any extra checks performed
through gtk_gesture_set_window() will be wrong, so the function has
been dropped entirely.