random: garbage collect the RANDOM_PURE_OCTEON entropy source

It was used for Octeon MIPS and all producers have been removed
from the source tree.

Approved by: re (cperciva)
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D53146
(cherry picked from commit 283699338c5724253f721f4d0e488113dd49bd17)
(cherry picked from commit b3bf22d70eb580b0a03906b17442f3950686b23a)