Revert "viced: Enable NAT ping on hosts"

This reverts commit aafdc08cfc49da4c23ecd91f9e690fd70e95df55.

The fileserver-side "NAT ping" behavior has yet to be proven to be
helpful in situations with NATs. If the behavior is not helpful, this
generates potentially a significant amount of extra useless traffic.
So until it can be shown to what degree this is helpful, keep this
behavior out of the fileserver.

Change-Id: Ibf6718eb1d37b2a7e610617acc697f4ee398b89a
Reviewed-on: http://gerrit.openafs.org/10712
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>