Belatedly document OPIE removal in RELNOTES and UPDATING.

MFC after: 3 days
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D41822
Approved by: re (delphij)

(cherry picked from commit d2c839eee0d5410095f5984d2126f41f4cf62a00)
(cherry picked from commit 2e38377fcda2eebb2f76f7f82352643a050d736e)