engine: Consider removed bonds in CollectVdsNetworkData

Changes to removed bonds should also be considered in the
CollectVdsNetworkData command, because they won't necessarily be torn
down by VDSM, in which case their label changes should still be
persisted.

There existed code to clear labels from removed bonds in
SetupNetworksCommand, but it was pretty much dead code, as removed
bonds by definition don't appear in the passed interfaces collection.

Change-Id: I44f1d5c0702e5062fe29bc89e12ff2400b46921b
Bug-Url: https://bugzilla.redhat.com/1085819
Signed-off-by: Lior Vernia <lvernia@redhat.com>