jaildesc: remove desc from the sysctl parameter list

Like lastjid, desc should count as a pseudo-parameter. The difference
lies entirely in the security.jail.param sysctls, which list all of
the jail parameters. Since desc opens and returns a file descriptor,
it has no place in such uses as "jls all." Like lastjid, it's still
recognized by the kernel if passed in/out, and hard-coded into libjail
so it can be recognized there.

MFC after: 3 days