ifconfig: remove debug printfs from set80211vhtconf()

Anyone testing VHT options would wonder about these extra two printfs
by now. Remove them from the tree before I have to do so locally again
in another branch.

Sponsored by: The FreeBSD Foundation
Fixes: e9bb7f9aa1b4f
MFC after: 1 week
Reviewed by: adrian, emaste
Differential Revision: https://reviews.freebsd.org/D48319