Drop 'Set to' from most src.conf(5) knobs

The description is clearly what effect the knob has when set, so the
additional text was unnecessary.

Reviewed by: jhb, se
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29583

(cherry picked from commit 9d178c925fb9acd88d2e5c5145d639b30b398c12)