Remove the remnants of SI_CHEAPCLONE

SI_CHEAPCLONE was introduced in r66067 for use with cloned bpfs. It was
later also used in tty, tun, tap at points. The rough timeline for being
removed in each of these is as follows:

• r181690: bpf switched to use cdevpriv API by ed@
• r181905: ed@ rewrote the TTY later to be mpsafe
• r204464: kib@ removes it from tun/tap, declaring it unused

I've not yet been able to dig up any other consumers in the intervening 9
years. It is no longer set on any devices in the tree and leaves an
interesting situation in make_dev_sv where we're ok with the device already
being set SI_NAMED.