restapi: Drop ip--mtu from update host's nic api

There mtu is not a property of the IP element.
Also, the update nic api doesn't support updating
the nic's mtu directly.

Change-Id: Iaafd159d6b489458ef8366f29838edf94b1afb28
Bug-Url: https://bugzilla.redhat.com/1049611
Signed-off-by: Moti Asayag <masayag@redhat.com>