ofagent: Remove network_delete method

Removing local_vlan_map entry here makes later port_unbound no-op.
While it isn't a problem right now, it will be when per-port cleanup
is introduced.
This commit simply removes network_delete method.  The local_vlan_map
will be cleaned up when the last port on the network is removed.

Related: blueprint ofagent-merge-bridges
Change-Id: Ib1f1ee16e0178d0362ff2b731957f1f52dd9a596
Merge "ofagent: Remove network_delete method"