Use winlink_remove() to remove old winlinks when synchronizing grouped sessions

rather than doing it manually and not adjusted the reference count. Fixes
crash seen by Dan Harnett.

session.c

@@ -589,7 +589,6 @@ session_group_synchronize1(struct session *target, struct session *s)
 	/* Then free the old winlinks list. */
 	while (!RB_EMPTY(&old_windows)) {
 		wl = RB_ROOT(&old_windows);
-		RB_REMOVE(winlinks, &old_windows, wl);
-		xfree(wl);
+		winlink_remove(&old_windows, wl);
 	}
 }