NSX: remove unnecessary checks on network delete

Since commit b50e66f router interfaces are not deleted
automatically when the network hosting them is deleted.

The NSX plugin still has additional logic to ensure all the
corresponding backend resources have been deleted as well.
Such logic is now obsolete and can be removed. Moreover it
also wasteful as it perform queries for retrieving NSX
resource identifiers.

This patch also amends a few related log statement to make
them clearer.

Change-Id: If3d8d98096a8363b216707f2558e37c80f58e515
Closes-Bug: #1339401
Merge "NSX: remove unnecessary checks on network delete"