Created attachment 1449994 [details]
screenshot

Description of problem:
[UI] - Remove subnet sub tab from edit external network flow.

Someone has added the subnet sub tab to the edit external network flow, it wasn't there and should not be there. Trying to create new subnet via the edit network flow doesn't work and doesn't do anything.

Creating subnets for external networks should be done via new network flow or via subnet sub tab under the network entity itself. 

Version-Release number of selected component (if applicable):
4.2.4.2-0.1.el7_3

How reproducible:
100%

Steps to Reproduce:
1. Create new external network with subnet
2. Edit the network

Actual results:
Subnet sub tab exist and doesn't do anything.

Expected results:
Subnet sub tab shouldn't be there. The only sub tab that should be under edit network is General.