ktls.4: Remove an obsolete statement

The default mb_use_ext_pgs value was toggled in commit 52cd25eb1aa.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30393

(cherry picked from commit c235059bb7e600b7bb88234836102fa9911addc2)