main/busybox: disable ash sleep builtin

The builtin has several known issues and has therefore been disabled upstream recently, let's do the same downstream. See:

• https://git.busybox.net/busybox/commit/?id=5e0411a7fb510b9aecda0a850c76bdd62c50efa4
• https://bugs.busybox.net/show_bug.cgi?id=15619

Instead, rely on the sleep BusyBox applet (CONFIG_SLEEP)