L3 agent supports multiple external networks from a long
time ago, so remove this RPC call since it is not used.
According to codesearch of [1] and [2], we just remove
neutron built-in L3 agent RPC. For neutron server side
or RPC callback classes, the function is still remained.
[1] http://codesearch.openstack.org/?q=get_external_network_id
[2] http://codesearch.openstack.org/?q=L3RpcCallback
Change-Id: I764423e175d6e82729a647e415a9f267f495916f
Closes-Bug: #1844168