compositor: Remove redundant and not well-defined focus field

It was never clear what this field really did.
diff --git a/src/compositor.c b/src/compositor.c
index 59a5abd..3a028bc 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -1042,7 +1042,7 @@
 {
 	struct weston_seat *seat;
 
-	if (!compositor->focus)
+	if (!compositor->session_active)
 		return;
 
 	wl_list_for_each(seat, &compositor->seat_list, link)