libweston: Remove functions with no implementation/definition

Seems that these functions: weston_compositor_fade()/weston_compositor_unlock()
lost their implementation a while ago.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

include/libweston/libweston.h

@@ -1750,12 +1750,8 @@ weston_output_damage(struct weston_output *output);
 void
 weston_compositor_schedule_repaint(struct weston_compositor *compositor);
 void
-weston_compositor_fade(struct weston_compositor *compositor, float tint);
-void
 weston_compositor_damage_all(struct weston_compositor *compositor);
 void
-weston_compositor_unlock(struct weston_compositor *compositor);
-void
 weston_compositor_wake(struct weston_compositor *compositor);
 void
 weston_compositor_offscreen(struct weston_compositor *compositor);