We don't really need to filter these out, it was just a leftover
safety check to not override the GDK_POINTER_MOTION_MASK.

Furthermore when we changed behaviour to not always select for native
pointer motion it is actually wrong. We'll still get normal motion
events for the toplevel which we will emulate as button motion on the
child, but the button motion mask will not be inherited by implicit
grabs which makes us not get any motion events during grabs.

This fixes bug 601473