wayland: do not generate motion events

Mutter generates a motion event for every button and scroll events,
which confuses Xwayland apps that rely on XMotionEvents for various
purposes, e.g. it fools rxvt jumpy mouse detection code.

Remove the call to notify_motion() from the button and scroll event
handlers to avoid these spurious motion events.

https://bugzilla.gnome.org/show_bug.cgi?id=748705