Remove cap_random(3).

Now that we have a way to obtain entropy in capability mode
(getrandom(2)), libcap_random is obsolete. Remove it.

Bump __FreeBSD_version in case anything happens to use it, though I've
found no consumers.

Reviewed by: delphij, emaste, oshogbo
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21033