libafs: disable mtu discovery

libafs: disable mtu discovery

we need to rework this to use lack of soft acks instead of this
method, which is too fragile

Reviewed-on: http://gerrit.openafs.org/6256
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 4d76b19b600aad461ee1231eeadb9b7a27b7f117)

Change-Id: Iba3f3d9d475959f99759db9e81c05c300aa6cd02
Reviewed-on: http://gerrit.openafs.org/6327
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>