Fix UAF when granting an output lease

Wlroots now destroys the wlr_output when granting a lease.
So we can't iterate through the outputs in the request after
granting the lease. This is also not necessary anymore because
they are already destroyed and thus removed from the layout.

include/labwc.h

@@ -433,7 +433,6 @@ struct output {
 	struct wl_listener frame;
 	struct wl_listener request_state;
 
-	bool leased;
 	bool gamma_lut_changed;
 };