core: Allow Removal of Network on Host from DC

This patch removes the canDoAction that checks
whether a network that is being removed from a
data center is used by one of its hosts.

This creates uniform behaviour, since detaching
a network from a cluster that has a host that
uses the network is allowed.

Change-Id: Ie66fc9ee6ebbc9a907bca4b1925bffaaeb111725
Bug-Url: https://bugzilla.redhat.com/909820
Signed-off-by: Muli Salem <msalem@redhat.com>