epoch(9): Remove the under-development note

There has not been planned changes so far to the interface. Remove the
section as it may not be relevant anymore.

Sponsored by: Juniper Networks, Inc.
Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D44501

(cherry picked from commit 04b90ac0f9e883a485bad4b0edf516395596d17f)