disable amd(8) by default

As of FreeBSD 10.1 the autofs(5) is available for automounting, and the
amd man page has indicated that the in-tree copy of amd is obsolete.
Disable it by default for now, with the expectation that it will be
removed before FreeBSD 13.0.

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22460