Remove setkey(), encrypt(), des_setkey() and des_cipher().

The setkey() and encrypt() functions are part of XSI, not the POSIX base
definitions. There is no strict requirement for us to provide these,
especially if we're only going to keep these around as undocumented
stubs. The same holds for des_setkey() and des_cipher().

Instead of providing functions that only generate warnings when linking,
simply disallow linking against them. The impact of this is relatively
low. It only causes two leaf ports to break. I'll see what I can do to
help out to get those fixed.

PR: 211626